This PR simplifies `Entity` to make only one way to add/remove components. ## Changes - Removes the `components` object on Entity as a way of adding/removing components - Extracts initializing entities with a template entity into a separate method, supports templates with children - Support cloning entities with children - Rename's `add()` to `addChild()`, `remove()` to `removeChild()`, and `removeAll()` to `removeAllChildren()`