protocol: disallow re-using wl_data_source

As pointed out in [1], re-using a wl_data_source for multiple start_drag or
set_selection requests has bad consequences, because this object has events
that allo tracking the state of a selection/drag-and-drop operation. Tracking
two operations at the same time isn't possible with this interface.

[1]: https://lists.freedesktop.org/archives/wayland-devel/2019-January/039936.html
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