x11: Allow X11 clients to clear the selection

According to the XSetSelectionOwner libX11 documentation:

  [...] If the owner window it has specified in the request is later
  destroyed, the owner of the selection automatically reverts to None,
  but the last-change time is not affected.

This is indeed visible through the selection_timestamp field in
XFixesSelectionNotify events.

Use this to check whether the selection time is recent-ish (thus
likely coming from an explicit XSetSelectionOwner request) and honor
the client intent by setting a "NULL" owner. If the selection time
is too old, it's definitely an indication of the owner client being
closed, the scenario where we do want the clipboard manager to take
over.

This fixes two usecases:
- X11 LibreOffice / WPS clear the selection each time before copying
  its own content. Mutter's clipboard manager would see each of those
  as a hint to take over, competing with the client over selection
  ownership. This would simply no longer happen
- Password managers may want to clear the selection, which would be
  frustrated by our clipboard manager.

There's a slight window of opportunity for the heuristics to fail
though, if a X11 client sets the selection and closes within 50ms, we
would miss the clipboard manager taking over.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1206

(cherry picked from commit 5671f0a2)