Xwayland on demand preparations

This is the Google Summer of Code branch rebased and improved upon, that prepares mutter for making Xwayland optional and on-demand. This is a continuation of https://bugzilla.gnome.org/show_bug.cgi?id=759538

This needs branches for gnome-shell and gnome-shell-extensions as well. Find them here: gnome-shell!86 (merged), gnome-shell-extensions!35 (merged)