This role defines a strategy (strategy pattern) encapsulating the algorithms
to determine the appearance of a prop used in a board game.
Create an instance of this strategy and supply it to the BoardDrawing so it
can find the proper image to set on a prop (BoardFigure) once the underlying
game emits a propChangeEvent.
Example: In backgammon, when a die is rolled, the game instance should emit a
propChangeEvent for that particular die. The BoardDrawing will listen/observe
the propChangeEvent and for the given die, request an instance of this
interface to generate the die image to display on the GUI.