app: GtkAction -> GAction madness part two

Change all action callbacks so they can be invoked by a GAction:

- add GimpActionCallback typedef:
  void (* cb) (GimpAction*, GVariant*, gpointer)
- change all action callbacks to the GimpActionCallback signature
- add "gimp-activate" and "gimp-change-state" signals to GimpAction,
  with the same signature as the resp. GAction signals
- remove all other custom action signals and only use the new
  GimpAction signals
- pass around appropriate GVariants containing booleans, int32,
  strings
- badly hack around to force a GimpProcedure pointer into a
  uint64 variant
- remove all G_CALLBACK() casts from all action callbacks,
  they all have the same signature now

(cherry picked from commit 3b6b3fc189)

app/actions/help-actions.c

@@ -40,13 +40,13 @@ static const GimpActionEntry help_actions[] =
   { "help-help", "gimp-prefs-help-system",
     NC_("help-action", "_Help"), "F1",
     NC_("help-action", "Open the GIMP user manual"),
-    G_CALLBACK (help_help_cmd_callback),
+    help_help_cmd_callback,
     GIMP_HELP_HELP },
 
   { "help-context-help", "gimp-prefs-help-system",
     NC_("help-action", "_Context Help"), "<shift>F1",
     NC_("help-action", "Show the help for a specific user interface item"),
-    G_CALLBACK (help_context_help_cmd_callback),
+    help_context_help_cmd_callback,
     GIMP_HELP_HELP_CONTEXT }
 };