Manipulating2D is a simple interface for user interface objects which can manipulate
portrayals in certain ways. The primary function of the Manipulating2D interface is
to pass Display2D etc. through to SimplePortrayal2D subclasses which implement the
handleMouseEvent(...) method without providing them an explicit class like Display2D.
Some of these classes (notably MovablePortrayal2D) need a few Display2D functions
to do their dirty work, which is where the Manipulating interface comes in. Eventually
as we determine that more Display2D features need to be made available to certain
SimplePortrayal2D subclasses, we may extend this interface.