sharing: Add Wayland screen sharing support

This allows enabling screen sharing support under Wayland, as long as
the org.gnome.Mutter.RemoteDesktop D-Bus name is exported by mutter /
gnome-shell. That name will currently only be exported if the
corresponding experimental features has been added to mutter.

So far nothing is configurable, as gnome-remote-desktop is yet to be
configureable.

https://bugzilla.gnome.org/show_bug.cgi?id=786565