Diagram Editor: support orthogonal routing of “incoming” and “outgoing” edges

[till-f]

"incoming" and "outgoing" edges means edges from/to parent box

[slisson]

The ELK "layered" layouter doesn't support this. Even after the refactoring that runs the layouter on the whole diagram hierarchy it still doesn't produce control points for them.

The layouter is implemented in the class "ElkLayouter". Maybe is missed a configuration parameter.

[alexanderpann]

I don't quite understand this ticket. If we are talking about the merging of edges from the same dummy port this should be covered in #844. It will also make it possible to directly connect one box to another without those ports.