Improve Wayland support
Freedesktop specifications mandate that the definition of XDG_SESSION_TYPE should be respected, so set it to 'wayland'
Tell the known toolkits that support Wayland to use the wayland backend setting GDK_BACKEND, QT_QPA_PLATFORM, CLUTTER_BACKEND, SDL_VIDEODRIVER and MOZ_ENABLE_WAYLAND.
Then use dbus-run-session launch labwc to make sure session with labwc starts as D-Bus session.
Lastly, make xfce4-session to use an exclusive and specific environment and rc.xml files for labwc.
Closes: #209
Edited by Gaël Bonithon