Implemented basic version of XfdashboardFitBoxLayout which layouts children like ClutterBoxLayout but takes care to fit into parent actor's width or height depending on orientation. It will alway set width or height of every child to match parent's size but it can scale it to keep aspect ratio if wanted. When implementing this layout manager also some minor errors and misbehaviours of some other actors were corrected (and hopefully throw no warnings or criticals anymore).