hgcia: access status fields by name rather than index

context: store status class instead of plain tuple in self._status This improves readability a bit by allowing us to refer to statuses by name rather than index.

status: update and move documentation of status types to status class The various status types are currently documented on the dirstate.status() method. Now that we have a class for the status types, it makese sense to document the status types there instead. Only leave the bits related to lookup/unsure in the status() method documentation.