Add separate edge creation handles

[pcdavid]

This is a first subset of #2443 which will only add the edge creation handles.
These will be different from the handles (black dots) used as connection points between edges and nodes.
They will only be visible when a node is selected (if the node can have edges), and will only serve to initiate the edge creation process.

The implementation will be based on the part of PR #2474 which corresponds to this subset.