wayland: Cancel selections if set whilst we don't have focus
Otherwise we confused clients in the following situation:
Client A sets selection, but due to timing has lost focus User copies something in client B Client later tries to paste in client A.
Client A still thinks it owns the selection so does nothing.
Tested with keepassxc which clears the selection with a new clipboard 10s after copying during which the focus is probably lost. Pasting into keepassxc was then broken.