extract scene into separate crate

[cyrgani]

As said in #768:

Honestly, scene overall needs to go. It should be either heavily refactored or be extracted into a separate crate.

This PR does that by removing the scene logic from macroquad and only adding a small handler that the new macroquad-scene crate can use to register a few functions so that macroquad can interact with the scene as before.
Also moved were the scene-related tests, the state_machine module and the CapabilityTrait derive macro.

A simple implementation of the proposed new macroquad-scene crate can be found here: https://github.com/cyrgani/macroquad-scene

Supersedes #795 and #796 (and likely some more), fixes #723.

[not-fl3]

I do not want to touch anything in scene/coroutines until macroquad-0.5, and macroquad-0.5 is not goint to have any of this anyway.

[cyrgani]

Will there be a replacement for it in 0.5?

[not-fl3]

I don't think so.
Just as you said, it should be a crate!

[cyrgani]

I'll keep this waiting until 0.5 is near, then.