Add support for wayland clipboard

I'm running Debian on a VM using UTM on MacOS.

Clipboard sharing does work between MacOS and Gnome (Wayland) because Mutter (Gnome's WM) handles both x11 and Wayland clipboard back ends and unifies them.

However, clipboard sharing does not work between MacOS and Sway (my WM of choice) because it doesn't do what Mutter does. It would be nice to see proper Wayland support added to spice-vdagent.

Related:

• https://unix.stackexchange.com/q/698083/313788
• https://www.reddit.com/r/swaywm/comments/pg0rqi/clipboard_sharing_using_spicevdagent_not_working/
• https://www.reddit.com/r/swaywm/comments/mwywls/clipboard_sharing_between_host_and_qemukvm_guest/