Simpler and more flexible scene model

[JakobKallin]

The current scene model consists of two hardcoded layers, one instance of each media type per layer, and mixins. Based on my own experiences, this model is both more limited and more complex than it should be.

• It's more limited because two layers aren't always enough, and because we sometimes want multiple sounds playing simultaneously in a single scene (a notable example being music and environmental sounds combined dynamically without having to mix a custom sound file).
• It's more complex mainly because of the mixin concept, which can be used to work past some of the limitations mentioned above, but still relatively seldom given its complexity. Almost every time I've used it, it has felt like a tedious workaround.

I have some ideas for to improve on both of these aspects but haven't made any final decisions yet.