kiosk-shell: Add transform_handler to correctly position xwayland surfaces

When using xwayland surfaces and multiple outputs we need to notify
xwayland surface that the surface position has changed, otherwise we're
going to end up with pop-ups being displayed on other outputs rather
than the one were the main surface resides.

Stolen from desktop-shell.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>