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/display-private.h

@@ -146,7 +146,7 @@ struct _MetaDisplay
   MetaEventQueue *events;
   GSList *screens;
   MetaScreen *active_screen;
-  GHashTable *window_ids;
+  GHashTable *xids;
   int error_traps;
   int (* error_trap_handler) (Display     *display,
                               XErrorEvent *error);