Add model reload button

[rmsc]

This is very useful when you're iterating on a model and don't want to keep going through the delete->load->reposition loop.

It was not as straightforward as I expected, so I've split it into several atomic commits to make it easier to review.

I'm not particularly happy with how supports track their parent model, but a cleaner implementation would likely need some deeper changes (maybe to the Project struct, where models are currently held in a Vec<Models>). I'm definitely open to suggestions.

[connorslade]

This is a really good idea. Right now support generation is still in development and not usable, so I don't think its worth worrying about tracking the supports.

Also, I'm not sure how I feel about serializing the file paths. They should probably be stored relative to the project file, but you may not always want the path embedded at all. Maybe there should be a checkbox to save it, or maybe it should only be saved if the model is in the same directory as the save.