Improvements to port label spacing

[soerendomroes]

Port label spacing is not set by the node that the port belongs to but by the parent of that node, which is bad but should be overridden by individualSpacings if desired.

Vertical and horizontal label port spacing is broken for negative values.

Port label placement should support always up and always down instead of always the same side. A better name would of course be left of elk direction and right of edge direction

[soerendomroes]

maybe we also want to be able set the port label spacing on a port and override the default set by the node. This should be done by individualSpacings.

[soerendomroes]

• [ ] Labelport spacing can be negative to solve all kinds of placement problems for very special ports
• [ ] Instead of ALWAYS_SAME_SIDE port label side should be configurable for each port direction (which may be overridden by individual spacing)
• [ ] It still seems to be unintuitive that the parent node is responsible for the port positions of all child nodes. This should be solved in a major version, since it drastically changes how ports are defined.