Bug 927862 - Implement AttentionWindow (WIP for v2.1)

My first intuition would be to let the `AttentionWindowManager` do the `setClip` on all instances.
But I know you don't want coupling :)

Can we find a compromise?
Maybe a mixin to maintain an array of all AttentionWindow instances, I know we could probably use one for the AppWindows too... And maybe one just to maintain the activeApp, just dreaming :)

But let's try to do something for `AttentionWindow` in the scope of this patch.