Add demands attention support to the messagetray

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