Add robustness against property manipulation

[ghostcity]

Some of the classes are not robust against manipulation. For example, if collections are given as parameters into getter-methods or are returned by setter-methods, these collections can be updated afterwards. Doing so will manipulate the holder of such collections.

I'd like to provide some fixes for that.

[damianszczepanik]

That might be true as this is not the main goal of the project :) I'm happy to see fixes for that!