Property-value redesign

Hi all, this is the redesign that store properties as datasets instead of groups. 
As said in  issue #220 it looks better in the . I for now decided to not support deleting of values.  
Since this pull request would introduce some changes to the API (e.g. Section::createProperty) it would be time for a tag before possible merge, isn't it?!

Excuse the merge commits due to  rebasing, etc... I gave up to clean that...