Force processing of ownerchange event when releasing the clipboard

Force processing of ownerchange event when releasing the clipboard

Make sure we process the XFixesSetSelectionOwnerNotify event caused by
us setting the clipboard owner to none, directly after setting the owner
to none. Otherwise we may end up changing the clipboard owner to none, after
it has already been re-owned because the XFixesSetSelectionOwnerNotify event
to owner none is event is still pending when we set the new owner, and
then changes the owner back to none once processed messing up our clipboard
ownership state tracking.

I saw this happening when doing copy twice in succession inside the guest.

+ some other misc fixes (memory leak, unneeded XFlush)