The source field was removed from ClutterEvent with commit
b644ea1bcebfef2fb3ed4b6b7de602000cece33b because the preferred way of
getting the event actor is now to use the device/sequence actor from the
stage directly.
With crossing events it's not that easy though, as crossing events
explicitly have a source and related actor that doesn't have to be the
same actor as the device actor. Since we kept around the "related" field
there anyway, let's also introduce a "source" field in
ClutterCrossingEvent and return that actor when get_source() is called
on a crossing event.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2551>
4fce0b974b