GdkDisplay: handle multiple calls to _gdk_display_pause_events()

Since events can be paused independently for each window during processing,
make _gdk_display_pause_events() count how many times it is called
and only unpause when unpause_events() is called the same number of
times.

https://bugzilla.gnome.org/show_bug.cgi?id=685460

gdk/gdkevents.c

@@ -88,7 +88,7 @@ _gdk_event_queue_find_first (GdkDisplay *display)
   GList *tmp_list;
   GList *pending_motion = NULL;
 
-  if (display->events_paused)
+  if (display->event_pause_count > 0)
     return NULL;
 
   tmp_list = display->queued_events;