Refactor namespaces and folder structure

Move things around to more clearly separate concerns.

The idea is to: 
- have a `core` folder for all things central to the domain of the application, including interfaces for rendering
- put graphics-specific stuff under `graphics`
- put different scenes in `scenes`, with only the main file directly under `scenes` and all implementation details in a separate namespace and directory, e.g. `src/scenes/julia` and `mossman::scenes::julia`