New C++ code generator from API metadata

- A recent PR in neovim introduces the api-msgpack-metadata option
  that can be used to retrieve API metadata - this allows us to
  get the metadata without parsing the Neovim source code
- Ported code generator to C++ that reads metadata blob from
  stdin
- Also some additional refactoring as a consequence
  * Function() internal data types are now QString instead of
    QByteArray
  * Moved parsing code out of NeovimConnector() into Function()
    class
- New CMAKE target **neovim_auto** calls the generator to create
  the c++ code