Use composition instead of inheritance for different node types

[eikel]

Initially reported by @ClaudiusJ as #596:

This is just a collection of crazy ideas that might be useful for MinSG 2.0. (I don't know if we should really consider this. Ralf came up with this interesting idea and I wanted a place to collect some thoughts on it.)

• All nodes can contain children
• To create a camera node, attach a camera attribute to a node; same for lights
• Meshes are also attributes (perhaps with a flag for faster access NODE_CONTAINS_MESH)
• Transformations could also be optional; only a world matrix and world bb should be contained for efficiency.
• Dynamic data structures should be possible using the existing observer concept.

Problems:

• bounding boxes: How are the bounding boxes of a mesh automatically used for the node, if the mesh is just an attribute?
• bunch of work for only a small benefit

Benefits:

• Slightly more flexible
• Nicer concept (for my taste)
• No virtual inheritance needed
• Early stages are compatible with the existing implementation: One could easily just use the new concept inside of the existing nodes.