Remove usage of MetaScreen
As of the libmutter API version 3 MetaScreen does no longer exist. Functionality that previously depended on MetaScreen has been moved elsewhere (e.g. MetaDisplay or MetaWorkspaceManager etc).
https://bugzilla.gnome.org/show_bug.cgi?id=759538
This needs mutter!95 (merged) and gnome-shell!86 (merged)