display: clean up focus_window vs expected_focus_window

Mutter previously defined display->focus_window as the window that the
server says is focused, but kept display->expected_focus_window to
indicate the window that we have requested to be focused. But it turns
out that "expected_focus_window" was almost always what we wanted.

Make MetaDisplay do a better job of tracking focus-related requests
and events, and change display->focus_window to be our best guess of
the "currently" focused window (ie, the window that will be focused at
the time when the server processes the next request we send it).

https://bugzilla.gnome.org/show_bug.cgi?id=647706

src/core/window-private.h

@@ -277,7 +277,7 @@ struct _MetaWindow
   /* EWHH demands attention flag */
   guint wm_state_demands_attention : 1;
   
-  /* this flag tracks receipt of focus_in focus_out */
+  /* TRUE iff window == window->display->focus_window */
   guint has_focus : 1;
 
   /* Have we placed this window? */
@@ -590,9 +590,8 @@ gboolean meta_window_property_notify   (MetaWindow *window,
                                         XEvent     *event);
 gboolean meta_window_client_message    (MetaWindow *window,
                                         XEvent     *event);
-gboolean meta_window_notify_focus      (MetaWindow *window,
-                                        XIEnterEvent *event);
-void     meta_window_lost_focus        (MetaWindow *window);
+void     meta_window_set_focused_internal (MetaWindow *window,
+                                           gboolean    focused);
 
 void     meta_window_set_current_workspace_hint (MetaWindow *window);