osx: Implement windowDidResignKey

Added isHiding field to _ClutterStageOSX to allow windowDidResignKey
delegate to not order full screen window back whilst the full screen
window was being hidden.  This caused other application windows to be
hidden.  Also added code to keep hidden stage windows from being listed
in the application's Windows menu.

https://bugzilla.gnome.org/show_bug.cgi?id=655311