Refactor to put core modules in subpackage

[almarklein]

This just moves files around. No changes to the logic.

All our backends are in the library root, so they are easy to import from rendercanvas.xx import RenderCanvas, loop. But our core modules were also in the root, and I thought things were getting messy.

I considered moving all backend modules into a backends subpackage, and then doing some lazy loading mechanic with def __getattr__ in the __init__.py, but that made me feel itchy, so let's keep it simple:

• Core modules go in core/
• Public utilities go in utils/
• Context logic goes in contexts/
• Only backend modules go in the root, with __init__.py and _version.py being the sole exceptions.