wayland: Send clipboard offers to all data devices from the same client

Make the data device track the keyboard focus, and use that list to
forward the clipboard selection to all data devices from the same
client.

This is however not the case of DnD data offers, as the semantics
of multiple in-flight offers is unclear.

Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1250

(cherry-pick of commit 7e4e3714)

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1262