Child collections to game objects and vice-versa (DEF-1843)

Right now sub collections can only exist in the root of a parent collection. This limits their reusability a lot. To be able to child them in a game object is desired. This is possible to do in code, but not in the editor.

Optionally, and obviously more complex: It would also be a benefit to be able to place game objects and other collections as children to sub collections, the same way we can place game objects as children to .go game objects.

Added value: Flexibilty and Reusability of asset