Support hierarchical edges not connected to explicit external ports

[le-cds]

ELK Layered is absolutely brilliant when it comes to supporting external ports. However, the external ports have to be excplicitly defined in the KGraph to be laid out. If an edge is incident to a child node and its parent, but not conneccted to a port of the parent, the edge simply gets ignored. That should be changed by introducing dummy external port dummies.

Without ports:

With ports:

This used to be KIELER ticket KIPRA-1643.

[le-cds]

It remains to be discussed how such implicitly created ports will mingle with explicitly created ones in scenarios where port orders are fixed.

[uruuru]

Would it be an option to only support it up until FIXED_SIDE or if there are no explicit ports for that node and side? Otherwise detect it and throw some exception?

IIRC it's not possible to have a set of ports on a specific side with fixed order (the pre-specified ones) and let the others change their order to minimize crossings (the automatically created ones).

[le-cds]

Would it be an option to only support it up until FIXED_SIDE or if there are no explicit ports for that node and side? Otherwise detect it and throw some exception?

Sure, we could through an UnsupportedConfigurationException.