Allow to implement interface

[Matthias Schoettle]

When importing an implementation class that is an interface and specifying that a class implements that interface, all methods of that interface need to be implemented.

The tool should provide the user to allow to do this. Perhaps, only the methods of interest can be implemented (however, then the code generator needs to add the other ones with no method body). One possibility in the UI could be to add an option to the ... button (that is used for constructors right now).

The code generator should further add the @Override annotation for Java code.