xwm: Fix selection if no seat present at startup

It's possible that there are no seats present at startup (especially with
the RDP backend, which creates and removes seats for connections), and
previously we'd just fail to set up XWayland cut and paste properly.

We should set up a listener and find a seat when one becomes available -
but we also need to switch seats if ours is removed.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
Co-authored-by: Hideyuki Nagase <hideyukn@microsoft.com>