Added MenuControl class to the freemind.modes.mindmapmode package.
This is done to refactor the MindMapController class which is a large class.  Code will be moved from the MindMapController to the MenuControl class.
Renamed MenuControl class to MindMapMenuController.
This makes it more consistent with the naming of the other classes and with the fact that it is closely linked to the MindMapController class.
Also added a basic constructor.