Added "render filters" to AvatarContainer.

- Render filters allow to specify which direct sub-avatar is rendered
by an avatar and in which order.
- Filters can be triggered on demand and automatically when avatars are
added or removed.
- Filters are an interface.
- They can be useful for selective rendering based on visibility, on
selection, etc.
- They can also be useful for ordering the rendering process, like
avatars that need to be rendered specifically before or after others.