xwayland: add "tablet" into the tablet device names

Changes the device name from "xwayland-stylus" to "xwayland-tablet stylus".
This doesn't fully address #26 but it goes a little step into making it more
human-readable.

wayland/wayland#26



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>