gdk: Use application_id as xdg_shell's app_id if available

This (manually) reverts commit e1fd8772

This makes sure xdg_shell's app_id matches the DBus name of the the
appliation when using GtkApplication. It also allows the compositor to
derive the desktop file ID from the app_id:

   https://cgit.freedesktop.org/wayland/wayland-protocols/tree/stable/xdg-shell/xdg-shell.xml?id=298d888ac718eae57ff2245d373d4327074506ea#n598

Closes #653

gdk/wayland/gdksurface-wayland.c

@@ -1515,7 +1515,9 @@ gdk_wayland_surface_create_xdg_toplevel (GdkSurface *surface)
 
   impl->initial_fullscreen_output = NULL;
 
-  app_id = g_get_prgname ();
+  app_id = impl->application.application_id;
+  if (app_id == NULL)
+    app_id = g_get_prgname ();
 
   if (app_id == NULL)
     app_id = "GTK+ Application";