[xwl] Fix clipboard clearing after externally changed

Closed David Edmundson requested to merge work/clipboard into master

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 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.

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