madcad file format .mc

[jimy-byerley]

This issue is about creating a file format dedicated to madcad works.

uimadcad's only purpose is to edit a python script generating the models. This issue is not about to change this. However for complex models, it is often useful to do have some persistent caching and to have external ressources. Most of the time requiring to create a directory dedicated to the model.
This issue propose to create a sort of archive format replacing such directory, allowing to save both the script and the resulting objects (meshes and all kind of data).

goals:

•  cache final and intermediate results to avoid recomputing everything each time uimadcad is opening a file
•  store external data, like imported models
•  store the window layout and other file-specific settings
•  store views and animations