[feature] Documentation

Complete QML documentation for Nemo QML plugin
social, and hacking docs as comments in
socialnetworkinterface.cpp.

Includes some polishing and API breaks:
- In ContentItemInterface: isIdentifiable property renamed identifiable.
- In FilterInterface: matches became non-invokable.
- In SorterInterface: firstLesserThanSecond became non-invokable.

- Tool doctojson.py reads a doc file and directly integrates it in the
  json file.
- Tool generate.sh can make use of doctojson.py to update documentation
  from .doc file before generating the interface.