Move closer towards an Entity-Component model

Move the framework design closer to entity-component-system model/design pattern. The original idea was that current AbstractData types (i.e. ImageData, GeometryData, ...) become Entities, that could get glued together to components. Transformation matrices, camera setup, lighting setup, and probably other stuff would also become entities, that would then define how to visualize the component. Hence, the DataHandles in the DataContainer would store components.

Possible steps (besides others):

• Rename XyzData to XyzEntity
• Develop a component class that can nicely glue several entities together
• Get rid of complicated (non-primitive) properties: Use Entities (DataHandles) instead, which would make sharing much easier and further contribute to the Entitiy/Component concept

However, directly realizing a entity system like this would lead to quite complex code. This is the reason I did not implement it. Nevertheless, one should check whether it is possible to move closer towards this central game engine concept.