Decoration is a simple struct that implements IDecoratedAround.

IDecoratedAround is the interface for any type that provides "decoration" on its left and right side e.g.

IDecoratedMiddle is implemented by any type that provides "decoration" in the middle of its render, such as a 'x' or a '-' symbol on a checked button.

IWidget is implemented by any widget that contains exactly one exposed subwidget (ICompositeWidget) and that is decorated on its left and right (IDecoratedAround).