should be an optimization, but has no real performance impact and prohibits SessionModel from refactoring

Move abstract item from IShape2DView into the subclasses of IShape2DView and change it into the concrete item subclass

This removes the necessity of ROI to implement the "show" button, to see the masking effects.

In a further step the mask presenter (which uses ROI functionality) could be removed, which was the motivation for the refactoring/redesign in the context of "remove ROI".