app: use a lot of g_clear_object() and g_clear_pointer()

More than 2000 lines of code less in app/, instead of

if (instance->member)
  {
    g_object_unref/g_free/g_whatever (instance->member);
    instance->member = NULL;
  }

we now simply use

g_clear_object/pointer (&instance->member);

app/display/gimpimagewindow.c

@@ -528,11 +528,7 @@ gimp_image_window_dispose (GObject *object)
       private->dialog_factory = NULL;
     }
 
-  if (private->menubar_manager)
-    {
-      g_object_unref (private->menubar_manager);
-      private->menubar_manager = NULL;
-    }
+  g_clear_object (&private->menubar_manager);
 
   if (private->update_ui_manager_idle_id)
     {