ADDED: UI - Anchoring widgets centered.
ADDED: UI - Introduced new interface IRenderable implemented by labels and sprites.
CHANGED: UI - Renamed TextData class to Label and moved it from Rendering to UI namespace.
ADDED: UI - Depth sorting of UI widgets.
CHANGED: UI - Cleaned up UI factory to make creation of UI widgets more modular.
ADDED: UI - Inheriting depth of panels.
ADDED: UI - Panels can be anchored.
ADDED: UI - Showing and hiding UI widgets.
ADDED: UI - Inheriting panel visibility.