Add Wayland support

[pabloab]

Now clipnotify uses XFIXES extension to X11. Since the world is moving to Wayland would be great to support it.

[cdown]

See cdown/clipmenu#52 for the sibling issue in clipmenu. Realistically, I'll only implement it when I actually start using Wayland, which doesn't look like it will be any time soon. If anyone else wants to do it though, feel free.