Outsource hierarchical logic into a separate baseclass

[skaupper]

Methods for adding, removing and querying children are identical for (most) core classes.
Create a base class with basic implementations for adding removing and querying (the querying methods should be stubs. that will be a separate ticket).

It should also be possible for inheriting classes to somehow modify the adding/removing process. Scenes for example need to add their elements to the right collision layer too!