Model editing and importing

[descawed]

I've begun work on the ability to edit models and import new models. Here are the remaining features I'd like to complete:

• Write out model files - necessary for everything else
• Model swapping (try to fit one character's model onto another character's skeleton)
• Import textures as a single 256x256 image and do the necessary UV lookups and quantization to turn it into the correct format (and if we're doing this, we might as well also support doing the UV lookups to export the texture as a single image)
• Import glTF models

[descawed]

Model file writing added by #28