[xwl] Fix clipboard clearing after externally changed

1c2f23d3 swapped round things so we
manage a dataSource rather than a dataDevice which may or may not have a
source.

In introduced a bug on clear. We only want to clear the wayland's
clipboard if xwayland owns the current clipboard. Otherwise we reset the
clipboard if some other client sets the selection. There's also no need
to wait for this to go through from our internal client to the server
representation - we can just clear immediately