New taskbar and icon groups positionning

Fix the position of appli icons amongst other applis or launchers (especially, make the order constant when detaching/inserting appli icons, by using the age of the windows)
Sort appli icons by window and class age.
Enable a more souple positionning of the taskbar (after/before launchers, beginning/end of the dock)
Rewrite the way to group icons (add semi-separated groups)