RFC: Refactor/context-components

This PR does two things:
- Removes `pc.ApplicationContext` and replaces it everywhere with `pc.Application`
- Adds `pc.Entity#addComponent` and requires `new pc.Entity(app)` to take application as first argument

The change is backwards compatible for online projects by using the default Application if no argument is passed to Entity.

See `examples/load_model/index.html` and `examples/animation/index.html` for an overview of how the new API looks compared to before.

Comments in particular about new Entity/addComponent API welcome.