Added the DropDownList factory and creation methods to the Screen class.
Updated small parts of the UI setup code in Catch.
Changed the "MenuButton" class into the "StackButton" class, designed to work with an API on the StackedContainer class.  This has all of the same functionality as the MenuButton class but can now be re-used for other StackedContainers.
Minor documentation update.
Updated the destruction of some widgets with explicit children.  This fixes what would have been a double delete error.
Code styling update.
Created a couple of currently blank files that will house the new UI callbacks for Catch.
Changed the way LayoutStrategies are initialized in QuadRenderables.
Completed the initial conversion of the TabSet to the new UI system.