Feature: Panorama: add ability to attach another ModelPanel to model in ModelPanel

[Trico-Everfire]

Which component should be improved?

UI/UX

Describe your feature suggestion in more detail

Give panorama the ability to have a ModelPanel attach another ModelPanel to a model's attachment point similar to Hammer's parent + attachment_point. This'd allow for a similar to CSGO ability for player characters to hold guns or wear cosmetics. Also for various other ideas.