MessageTray: untangle click notifications

MessageTray: untangle click notifications

Previously, when you clicked on a notification, it would call
this.source.clicked(), which would emit a 'clicked' signal on the
source, and then various other stuff would happen from there. This
used to make a little bit of sense, when clicking on a notification
was supposed to do the same thing as clicking on its source, but makes
less sense now, when clicking on the source itself *doesn't* call
source.clicked()...

Change it so that when you click on a notification, the notification
emits 'clicked' itself, and the source notices that and calls its
notificationClicked() method, and the various source subclasses do
what they need to do with that, and Source no longer has a clicked
method/signal.

https://bugzilla.gnome.org/show_bug.cgi?id=631042