Minor changes in UI widgets

Hello!

As I have been using libGDX for over 2 years by now, [I] decided to make some contributions to common code base. Please find first minor changes to HorizontalGroup and VerticalGroup (to partially support zero-width widgets), and an addition to Actor class as this is something I repeatedly ran into and had to implement locally while developing gdx-based games.

As always, libGDX is simply terrific and indispensable. Great job guys!