-Introduction of two new interfaces that formalize communication between a component and its renderers: IItemRenderer, IItemRendererOwner
-Removed ISelectableRenderer from use by ButtonBar. ButtonBar now works with the above-mentioned interfaces.
-Added labelField/labelFunction to ListBase - usable by List and ButtonBar.
-Introduced new utility class, LabelUtils, which returns the correct text for a data item's renderer given a labelField and labelFunction.
-Removed data-binding from FxDefaultItemRenderer - the new interfaces determine how a component stuffs the right text into the renderer's optional part, labelElement.
Mustella: ButtonBar and List tests pass