Make port label placement more consistent

[uruuru]

#634 and #626 combined port label placement within a single layout option. Yet, the way the various configurations work is not consistent:

• inside and outside behave differently by default (inside even behaves differently for non- and hierarchical edges)
• always same side and space efficient do not apply to inside labels
• north/south behaves differently by default

Also see the comments in PortLabelPlacement.

[le-cds]

Another point is that inside port labels placed next to their port will move out of the way to avoid being crossed by an edge. Outside port labels don't seem to do that.

[lredor]

For information, yo have a a sample of inconsistent behavior in gitter