[GREENFOOT-676] Adding/removing a class causes recompilation of all classes, reset

Adding or removing a class, or editing a class which is not a dependency of the current world, currently cause recompilation of all classes and a reset (or at least discard) of the world. This seems counter-intuitive and causes unwanted noise in the interface. I think it would be much nicer if this could be changed.

Consider the use case of a user who is interactively adding objects into a world, then realises they need a new class. They create the new class without "saving the world" first - then all their effort is lost; there is no warning that this will happen and no way to recover.

Issue metadata

Issue type: Bug
Priority: Medium