GtkApplication: change the accel prefix we use

GtkApplication adds to the global accel map using the prefix <Actions>
which is also used by GtkAction.  This causes GtkApplicationWindow to
try to parse GtkAction-added accels as if they were its own (which
fails).

Switch to a different namespace -- <GAction>/.

https://bugzilla.gnome.org/show_bug.cgi?id=668367

gtk/gtkapplicationwindow.c

@@ -375,10 +375,10 @@ add_accel_closure (gpointer         data,
   if (accel_key == 0)
     return;
 
-  if (!g_str_has_prefix (accel_path, "<Actions>/"))
+  if (!g_str_has_prefix (accel_path, "<GAction>/"))
     return;
 
-  path = accel_path + strlen ("<Actions>/");
+  path = accel_path + strlen ("<GAction>/");
   p = strchr (path, '/');
   if (p)
     {