This drops all subclasses of `MessageTray.Source` that were used to
display system notifications. Now the `Source` returned from
`MessageTray.getSystemSource()` is used.
Ensure also that properties and methods that where set on the `Source`
are moved to the `Notification` object itself.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3156>