frames: Fix pressed state of window menu buttons

Since window menus have been moved to the compositor, the pressed
state of the corresponding window buttons is messed up, as it is
reset immediately when getting a LeaveNotify event due to the
compositor taking a grab. Fix this by ignoring that particular
event.

https://bugzilla.gnome.org/show_bug.cgi?id=731058

src/ui/frames.h

@@ -80,6 +80,7 @@ struct _MetaUIFrame
   int text_height;
   char *title; /* NULL once we have a layout */
   guint shape_applied : 1;
+  guint maybe_ignore_leave_notify : 1;
 
   /* FIXME get rid of this, it can just be in the MetaFrames struct */
   MetaFrameControl prelit_control;