Cache avatars to avoid having to re-download them every session.
We use a cache format that is compatible with empathy's, but we don't
actually use empathy's. This could be changed if we wanted.
https://bugzilla.gnome.org/show_bug.cgi?id=614974
Also reorganizes the notification layout to use an StScrollView; very
tall notifications are now scrolled instead of just taking up more and
more of the screen.
https://bugzilla.gnome.org/show_bug.cgi?id=608999
