Allow child nodes for images within sequence diagrams

[dunkelziffer]

Preventing further nesting inside of nodes with shape: image is a reasonable decision. However, containers with shape: sequence_diagram completely change the rendering of nested nodes. Suddenly child nodes get rendered below their parents instead of nested within them. Now having child nodes makes sense again for images.

The following should render correctly:

shape: sequence_diagram
alice: {
  shape: image
  icon: https://icons.terrastruct.com/essentials%2F365-user.svg
}
bob: {
  shape: image
  icon: https://icons.terrastruct.com/azure%2FCompute%20Service%20Color%2FCloud%20Services.svg
}
alice.a -> bob: request
alice.a <- bob: response

[alixander]

closed by #2061