Fixes NPE when closing before initializing TransitionManager.
MeshHelper frees all resources when it's no longer needed.
Merge pull request #812 from RotaruDan/npeFix

Fixes NPE when closing before initializing TransitionManager.
Adds ScalePanel as a panel with a scaling animation.
PanelOverActor: a ScalePanel that is positioned on top of an actor. Also adds a test.
DropDown now uses a PanelOverActor when is opened.
IconWithPanel refactoring. Adds IconWithPanelAbove.