Putting the notification actor in the tray actor has caused a lot
of various bugs and glitches over the years related to syncing the
two, fizzling out events, and so on. It's a much simpler model if
we consider the notification actor and tray to be separate widgets.
As a side effect, this makes the context menu not pop up when we
right-click on notifications.
https://bugzilla.gnome.org/show_bug.cgi?id=695800
8c32102e99