#68 Add support for hierarchical tags

[_duSt_]

Currently tags are treated independently and only a list is provided. It would be good to support tag hierarchies such that knowledge can be grouped and categorized.
This could be realized by treating a "." character in a tag as a hierarchy level separator. I.e. a tag named "Search.Index.Creation" would consist of 3 levels "Search" "Index" and "Creation" where each one of the 3 levels will be treated as an independent tag on the one hand, and as elements of a path within a tag hierarchy tree on the other hand.
The tag serach dialog should then also provide some kind of hierarchical display of the tags.