display: Rename window_ids to xids

As the hash table no longer stores only window IDs, we should rename it so
that we make sure to check if something is actually a window before using it
as a window.

https://bugzilla.gnome.org/show_bug.cgi?id=677215

src/core/screen.c

@@ -1220,7 +1220,7 @@ meta_screen_foreach_window (MetaScreen *screen,
    */
   
   winlist = NULL;
-  g_hash_table_foreach (screen->display->window_ids,
+  g_hash_table_foreach (screen->display->xids,
                         listify_func,
                         &winlist);