xwayland: set the app_id and install a desktop launcher

The app_id is used to identify applications (and group windows), some
desktops (such as GNOME Shell) use it in their top bar.

Set the XDG toplevel "app_id" to "org.freedesktop.Xwayland" and install
a desktop file for Xwayland rootful.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>