Create a StgitObject class to factorise code for property handling.

This change makes it easier to add new stored fields to the Series object,
without having to duplicate existing code from Patch.

Generic field accessors were taken from the Patch class.  Dir accessors
were added to avoid making the __dir attribute public, and were used
to replace Series::__series_dir (that name was a bit redundant anyway).
Create_empty_field came as a natural addition to factorise more code.

Signed-off-by: Yann Dirson <ydirson@altern.org>