WiP feat: link external label with its parent

[jarekdanielak]

Related to camunda/camunda-modeler#369

Proposed Changes

Show a dotted link between an external label and its parent when it's selected.

To do

• Find the proper layer to draw the lines on for non-root planes (e.g. expanded subprocesses)
• Find a better "middle point" for sequence flows with complicated shapes (not a straight line)
• Add tests
• Make the line more delicate

Try

Run npm start on this branch and try it out.

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

What you want to use is LineIntersection#getApproxIntersection(waypoints, reference) or a derivative. This gives you the closest anchor point, either on a bendpoint, or on the connection itself.

[nikku]

This looks great:

• self contained
• simple enough
• working well

Looking forward for the follow-up to get it into the tool. One thing to consider: Whether to make it an (optional) diagram-js feature. I see that other tools, i.e. dmn-js, may benefit from it, too.