Missing features and tests for Section 

This one is a bit larger than it was expected ...

Most changes are about streamlining the API of Section with existing implementations. Further several features, which were removed during the refactoring process, were added again:

* Recursive search through section trees
* Parent section access
* Links and inherited properties

I also introduced some changes to other parts of the code:
* The acceptAllFilter is now a template in the util namespace.
* ImplContainer has now const operator== and operator!=