Issue #12308: [macOS] dockable dialogues disabled on Windows menu.

Using the app. prefix/group for the top menu works better for the
GIMP_GTK_MENUBAR variant of the menubar (which is what is used on macOS
to get the macOS-style menu).

The whole action group code should be reviewed and straightened up
eventually!

menus/meson.build

@@ -56,7 +56,7 @@ foreach menu_filegen : [ 'dockable-menu.ui', 'image-menu.ui', ]
   if menu_filegen == 'dockable-menu.ui'
     group = 'dockable'
   else
-    group = 'dialogs'
+    group = 'app'
   endif
   conf.set('GROUP', group)
   included_file = configure_file(