As a further optimization (even though the big slowness situation is
mostly resolved anyway by a previous commit), let's not rebuild whole
submenus when only a few items were added or removed.
1. First make so that the action "visible" property is only notified
when the visibility actually changes. This way, we can trust that the
visibility of the item and of the related action are properly synced.
2. Now when an action which is part of a model changes its visibility,
we trigger the proper "items-changed" only for the added or removed
item. This replaces the old procedure of removing then adding
everything (i.e. a full reset).
3. Add some utilities in gimpwidgets-utils (out of gimpmenushell.c).
This cleans up some code and also fixes some bugs which were
reimplementing (badly) some code I already implemented (right) in a
static function.
4. Fix adding new actions in sub-sub… menus. Previous code was only
showing the direct parent, not up to the root menu (this could happen
for plug-in procedure items in particular).
When an action is removed from a group, we verify if it's not in any group
anymore. If so, we remove it from the GimpApp action map.
This fixes some CRITICALs happening when some actions were dynamically deleted
and re-created, in particular the "windows-display-XXXX" actions.
Since GAction don't have labels or visibility, it is up to every application to
fill the GMenuModel with such infos. In my previous implementation, I was simply
handling these in GimpMenuShell subclasses (GimpMenu, GimpToolbar and
GimpMenuBar) since we need them for tooltip support (unavailable from GMenu).
Nevertheless there are cases where we want to use GTK API directly with a
GMenuModel, in particular with gtk_application_set_menubar(). This is necessary
to handle the macOS specific code path, where we don't want our usual menu bar.
This OS has its own display of a software menu bar, directly in the desktop GUI.
See !558 for some screenshots of this.
So this commit moves around some code away from GimpMenuShell subclasses into
the new GimpMenuModel class. Now we use this new class instead of simpler GMenu
objects. It handles syncing with GimpAction-s, auto-updating labels, visibility
and sensitivity, as well as adding custom items (plug-in actions, recently
opened files, opened images, etc.).
