Lateral edge marker offset

[tomas-kucera]

This is a new feature suggestion.

Currently all edges (except for multiple edges between the same nodes) go from the center of a source node to the center of a target node.

It would be great to have the ability to laterally offset the marker. Similarly as the margin on the marker allows axial offsetting.

It could be simple as the multiedge situation allows this offset automatically and is also configurable with the gap property.