- remove unused ones 
- add the attribute background_format that permit to change the file format use
  for the background
- remove original_page_height/width because the were always set to 0 and never
  used


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@18933 20353a03-c40f-0410-a6d1-a30d3c3de9de