This is part of the design update for the message tray.
Source now takes an extra argument called 'title'.
All expanded message tray items are same width, which is determined by
the width of the item with the longest title, up to MAX_SOURCE_TITLE_WIDTH.
This is done so that items don't move around too much when one is expanded
and another one is collapsed.
https://bugzilla.gnome.org/show_bug.cgi?id=617224
Inform the user about demands attention events using the messagetray.
Clicking on the notification icon moves the user to the window.
It differentiates between newly started apps and windows of already running apps, by showing different banners for this cases.
It is based on Jon Nettleton's "window attention" extension.
https://bugzilla.gnome.org/show_bug.cgi?id=610594
