Is your feature request related to a problem?

This was originally reported by @marek-piirikivi on Matrix.

Diagrams that provide internal views of objects should have a border around them. While developing an IBD, I would like the ability to add ports on the outside border of the diagram to show interfaces with the Block that the IBD represents.

Describe the solution you'd like

Render a border around the outside of a diagram and allow ports to be placed on it.

Describe alternatives you've considered

@amolenaar mentioned on Matrix that we could remove the whole IBD and then have users draw internal to a block using a Block itself.

Additional context