Fix dagre panic

[gavin-ts]

Summary

Fixes a panic with dagre layout when there is an edge to a text shape container.

Details

• if a shape has a markdown label it is considered a text shape instead of having a label.
• when we check for edges running into container labels we didn't consider containers being shape text
• text shapes have nil LabelWidth and LabelHeight values which panic on dereference
• TODO further consider containers as text shape
  • hard to tell where the container boundaries are
  • maybe it makes more sense with a background color?
• fixes dagre panic #906
• adds test case

previously this would panic with dagre

[alixander]

okay as stopgap