Start of diff plugin - Update of dependency Tree - Change of config syntax

Starts the diff plugin - currently without any outside calls. We'll work on that separetely.
Implements the information flow between this plugin and the general tree represenation.

The dependencyTree directive has received substantial updates in the process and is more flexible and cleaner now.

Also contains breaking changes to the syntax of configuration files, which aren't compatible to the current confEditor design. A follow-up Pull Request will handle this - there we will also describe the changes we have made here.
This PR is too large at it is anyway - and others might have a use for the changes it brings.

The navigator is temporarily disabled as well, as its current design stands in the way of reusable retrievers. This is a topic of ongoing discussion in #64.