Refactoring of TextDocument according base_convertable interface.

Now if a TextDocument is filledi in with a file, convertToBaseFormat
will try to convert the content of this file into utf-8 whatever
the encoding used.
The result of base_data conversion will not be stored into base_data property but
text_content.
Backward compatibility is keept, thanks to getContentType which reads text_format property
if content_type is empty.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@35242 20353a03-c40f-0410-a6d1-a30d3c3de9de