Objects implementing EventSource should have some mandatory methods and
properties, we can ensure this by defining an EventSourceBase abstract
class.
So inherit EmptyEventSource and DBusEventSource from it making sure that
they implement all the needed methods, using native properties and
replacing the 'notify::*' fake signal emissions with proper object
notifications.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/563
5e43f282a1