Feature request: allow label tags to be more than just a simple string

[jeff-phillips-18]

Describe the problem
I'm looking for help on implementing the user experience for editing edges. Our mockups suggest replacing the tag with a label that includes a context menu, allowing users to interact with the edge context menu to make edits.

Mockup screenshot:

[jeff-phillips-18]

/cc @marshmalien

[jeff-phillips-18]

Labels can be added by adding children to the edge rather than using the label property. I think it's a better approach than overloading with the label does.