Split GDK initialization from MetaUI

Get rid of the deprecated gdk_display_get_default_screen ()

https://bugzilla.gnome.org/show_bug.cgi?id=759538

src/core/main.c

@@ -599,10 +599,6 @@ meta_init (void)
     meta_fatal ("Can't specify both SM save file and SM client id\n");
 
   meta_main_loop = g_main_loop_new (NULL, FALSE);
-
-  meta_ui_init ();
-
-  meta_restart_init ();
 }
 
 /**