compositor: Add an API to query if the stage is focused

gnome-shell needs to know whether the stage window is focused so
it can synchronize between stage window focus and Clutter key actor
focus. Track all X windows, even those without MetaWindows, when
tracking the focus window, and add a compositor-level API to determine
when the stage is focused.

https://bugzilla.gnome.org/show_bug.cgi?id=700735

src/core/display-private.h

@@ -113,6 +113,9 @@ struct _MetaDisplay
    * or event that caused this.
    */
   MetaWindow *focus_window;
+  /* For windows we've focused that don't necessarily have an X window,
+   * like the no_focus_window or the stage X window. */
+  Window focus_xwindow;
   gulong focus_serial;
 
   /* last timestamp passed to XSetInputFocus */