wayland/data-device: Cancel drag source when the drag finished unsuccessfully
Quoting the first commit:
When a drag was performed but did not finish successfully, e.g. because no
mimetype was accepted, we need to send the source cancel event so clients
know they can destroy the drag source (since version 3).
and some small fixes.
Fixes #740 (closed)
@carlosg: that code might look familiar to you :P