build/linux: Make flatpak maintainer instructions clearer

Some parts of the README got obscured or dated with time. Let's improve them:

- Remove redundant text about keeping GIMP buildable (it's already said on
  Cod. Style and Dev rights pages on gimp-web-devel, and not flatpak specific)
- Remove text about 1-hour timeout which isn't true according to my tests,
  specially after the improvements of splitted jobs and updated build options
- Reorder Anitya text to before "module dropping" text, it's linear this way
  ("module dropping" by checking runtime is last step of module cycle of life)
- Remove very confusing "flatpak run... gimp" line which was breaking the
  explanation flow about "module dropping" and it's not too useful since the
  manifest (which is only a bit different) can be acessed in-source with tags
- Changed title from "Mantaining the manifest" to "Mantaining the modules"
- Added not so clear "Versioning the flatpak" to separate it from other modules
  text. The title is not perfect, but at least make a needed pause on reading

build/linux/flatpak/README.md

@@ -13,13 +13,9 @@ We recommend to look at the `README.md` file in respectively the `master` or
 Flathub does not host nightly builds, therefore we publish them on GNOME's
 Nightly repository. Our "nightlies" are actually "weeklies" through a [Gitlab job
 schedule named "Flatpak
-nightly"](https://gitlab.gnome.org/GNOME/gimp/-/pipeline_schedules).
+nightly"](https://gitlab.gnome.org/GNOME/gimp/-/pipeline_schedules), which builds
-
+whatever is on GIMP's repository `master` branch. The nightly manifest file is:
-This job will build whatever is on GIMP's repository `master` branch (this
+`build/linux/flatpak/org.gimp.GIMP-nightly.json`
-branch should be kept buildable and usable at all time, not only for scheduled
-jobs, but also for all contributors to be able to improve GIMP at all time).
-
-The nightly manifest file is: `build/linux/flatpak/org.gimp.GIMP-nightly.json.in`
 
 This file should remain as close as possible to the development manifest
 (`org.gimp.GIMP.json` file on the `beta` branch of the Flathub repository) which
@@ -33,14 +29,11 @@ Base rule to update the nightly build manifest:
 * Regularly `org.gimp.GIMP-nightly.json` should be diffed and synced with
   development and stable `org.gimp.GIMP.json`, in particular for all the
   dependencies (which are mostly the same across all 3 builds).
-* A merge request with the label `5. Flatpak package` will contain the `flatpak`
+* A merge request with the label `5. Flatpak package` will contain the `*flatpak*`
-  job, hence allowing theoretically to build a standalone flatpak (without being
+  jobs, hence allowing theoretically to build a standalone .flatpak (without being
-  published to the nightly repository) for MR code. In practice, jobs have an
+  published to the nightly repository) for MR code be tested.
-  1-hour timeout and our flatpak takes longer than 1 hour to build (there is an
-  exception in our repository, but only for the `master` branch), so we often
-  need to publish to `master` after mostly a visual review.
 
-## Maintaining the manifests
+## Maintaining the modules
 
 * GIMP uses Flatpak's [GNOME runtime](http://flatpak.org/runtimes.html), which
   contains a base of libraries, some of which are dependencies of GIMP.
@@ -54,41 +47,8 @@ Base rule to update the nightly build manifest:
   necessary. For more options, check [flatpak builder's manifest
   format](http://flatpak.org/flatpak/flatpak-docs.html#flatpak-builder).
 
-* On the other hand, if we increased the runtime version in particular (or when
+* Among the dependencies we self-build, some sources on GIMP manifest have set a
-  the `master` runtime evolves), some modules may no longer be necessary and can
+  `x-checker-data` property which makes it possible to check for updates using
-  be removed from our manifest.
-
-  A flatpak is a layered set of modules. Our GIMP build in particular is
-  built over the GNOME runtime, itself built over the Freedesktop
-  runtime, itself based on a yocto-built image.
-  Other than by trial and error, you can find the installed dependencies
-  by running:
-
-```sh
-flatpak run --devel --command=bash org.gnome.Sdk//master
-```
-
-Or if you already have a build:
-
-```
-flatpak run --devel --command=bash org.gimp.GIMP//master
-```
-
-Inside the flatpak sandbox, GIMP's manifest can be read with:
-
-```sh
- less /app/manifest.json
-```
-
-GNOME and Freedesktop's module lists (generated manifest as the SDK is built
-from BuildStream):
-
-```sh
-less /usr/manifest.json
-```
-
-* Some sources have set a `x-checker-data` property which makes it possible to
-  check for updates using
   [flatpak-external-data-checker](https://github.com/flathub/flatpak-external-data-checker).
   To run the tool either install it locally, via flatpak or via OCI image.
 
@@ -114,6 +74,29 @@ podman run --rm --privileged -v "$(pwd):/run/host:rw" ghcr.io/flathub/flatpak-ex
      reasons only, we might bypass this limitation, adding a comment
      explaining why we use an unstable release.
 
+* On the other hand, if we increased the runtime version in particular (or when
+  the `master` runtime evolves), some modules may no longer be necessary and can
+  be removed from our manifest.
+
+  After all, a flatpak is a layered set of modules. Our GIMP build in particular
+  is built over the GNOME runtime, itself built over the Freedesktop
+  runtime, itself based on a yocto-built image.
+  Other than by trial and error, you can find the installed dependencies
+  by running:
+
+```sh
+flatpak run --devel --command=bash org.gnome.Sdk//master
+```
+
+Inside the flatpak sandbox, GNOME and Freedesktop's module lists
+(generated manifest as the SDK is built from BuildStream) can be read with::
+
+```sh
+less /usr/manifest.json
+```
+
+## Versioning the flatpaks
+
 * For the development releases and nightly builds, we added the
   `desktop-file-name-prefix` property. For a stable release, the property line
   can be removed from the manifest.
@@ -126,6 +109,6 @@ podman run --rm --privileged -v "$(pwd):/run/host:rw" ghcr.io/flathub/flatpak-ex
   "gimp", "babl" and "gegl" modules, set "tag" to the git tag (ex:
   `GIMP_2_99_14`) and "commit" to the git commit hash for this tag.
 
-* For a nightly build, set top "branch":"master", and inside the
+* For a nightly build, set top `"branch":"master"`, and inside the
   "gimp", "babl" and "gegl" modules, set "branch" to "master", and
   remove any "commit" line.