[xwl] Cache KWaylandServer::DataSourceInterface as a QPointer

Seat correctly emits dragEnded when a DataSourceInterface is destroyed,
and stops the drag.

However WlToXSource lives slightly longer to finish handling any other
visits. See the guarded delete statement in Dnd::endDrag.

With recent fixes use is mostly a hypothetical, but we still don't want
dangling pointers in our code.

(cherry picked from commit 2d681965)