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>
Now that scripts are loaded as external modules, there's no reason
anymore for bundling them with the gnome-shell executable. Just
move the scripts into a dedicated folder in tests/ and run them
from there.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2812>