Make importing of operations possible

[Jörg Kienzle]

Currently it is possible to import classes (from extended models). However, it is not possible to import operations. It would be great if the circular menu that pops up when you tap-and-hold on a class would also include an “import operation” button in case the class is a ghost.

[Jörg Kienzle]

Actually, importing attributes would be nice too :)