[xwl] Fix clipboard clearing after external changes

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

It introduced a bug on clear.

We only want to clear the wayland's clipboard if our clipboard is emptied and we own the clipboard currently being used by the seat.

Otherwise we reset the seat's clipboard when some other client sets the selection.

(Long term now we have an AbstractDataSource that doesn't have to be a resource, I hope to kill our KWayland::Client internal connection usage, it'll simplify even more)