If a client is naive enough to assume that it can set the selection while
it has got no surfaces, mutter will simply ignore the request and leave
the selection unchanged.
This is good and the expected behavior, however the poor client that did
this will enter in an inconsistent state where it "claimed" the selection,
but nobody told it that the wl_data_source is not current.
So, when the client is focused the next time, it will receive wl_data_offers
as usual, but it will still think all the time that it is owning the
selection. In the case of GTK, that takes client-side shortcuts, so any
attempted paste will still bring back the client-side aborted selection.
To fix this, cancel the selection right away if it happened while unfocused,
the client will be able to undo its own failed selection, and not assume
that future offers are its own.
Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1469
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1772>
Instead of having everything clumped at MetaWaylandDataManager,
split the primary selection to its own struct. This manager is
handled separately from wl_data_device_manager and other selection
managers, so they would be able to interoperate between them, even.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1193
