[GREENFOOT-41] Move classes in the class browser

It would be nice to be able to move classes in the classbrowser. This can be divided into two cases:

Moving just for the sake of rearranging the order in which the classes are displayed
Moving a class to another hierachy \- giving it a new super class.

Changing the super class is now supported. Changing the super class in the editor will be reflected in the class browser

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