Create class

Can now create classes. Renamed the Class.cs to Node.cs in order to avoid confusion with generic type class.
Aded functionalty such that the class is "Smart" to drag, which means it stays where the cursor is.
Added a generic utility to convert bool values in xaml. Right now this is used to translate True/false into +/- based on the modifier on a class. Still we cannot undo/redo or add attributes and other things to one class. And the design is still flawed!

The way of using commands is still probably also wrong.