GdkDisplay: handle multiple calls to _gdk_display_pause_events()

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