messageTray: Chain up in NotificationPolicy constructor

messageTray: Chain up in NotificationPolicy constructor

We currently deliberately avoid chaining up in derived policy
constructors to not override properties with their defaults.
That's a neat trick that will stop working when porting to ES6
classes, as chaining up is necessary to actually initialize the
object there (including "this").

Address this by turning all properties into (overridable) getters
that are backed by private properties by default.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/350