build, plug-ins, gitlab: make GIMP runnable from build directory without…

… being installed.

There is already most of the main code logic for this, though now plug-ins need
to be in their own subdirectories, which breaks for plug-ins/common/ and
plug-ins/python/, while I needed plug-ins in both these categories to generate
the Windows installer welcome images (file-png, and python-fu-eval in
particular).

Once again, meson was not very helpful, since all its functions still refuse to
output generated files in subdirectories, so I end up duplicating plug-in files
with a custom Python script.

This should fix the CI. It was working on my machine as GIMP was installed, but
such a build rule should work even without GIMP installed.
This will also be useful in the future when we'll want to run unit tests of
plug-ins through the finale GIMP binary itself.

plug-ins/python/python-console/meson.build

@@ -1,5 +1,5 @@
 
 plugins += {
   'name': 'python-console',
-  'srcs': files('pyconsole.py', 'python-console.py'),
+  'srcs': [ 'python-console/pyconsole.py', 'python-console/python-console.py' ],
 }