This helps illustrate that expired notifications move to the summary view. Animate the separate components of the message tray individually as in http://www.gnome.org/~mccann/shell/mockups/20090630-demo Based on the patch from Florian Müllner.
This decouples Source and MessageTray, so Source doesn't need to know how MessageTray works.
https://bugzilla.gnome.org/show_bug.cgi?id=608058
From the message-tray branch. Most of the UI parts were written by Marina, and most of the D-Bus parts by me.
