Split IConvertable to IFormatConvertable and IConvertable. Move implementation from abstract Document class to interface (IConvertable) implementation. Adjust interface doc strings accordingly. 

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@37045 20353a03-c40f-0410-a6d1-a30d3c3de9de