- add a file format attribute that permit to chose the format ImageMagick will
  convert the background into (now it's easy to change in png or jpg, or 
  whatever convert could support)
- remove a lot of unused code
- remove dangerous code !
- enhance security on file deletion by removing only the files used during this
  method execution (and not using rm -f /tmp/tmp* like it was)
- use only one convert command (from ImageMagick) so the pdftoppm command 
  (from Xpdf) it's not yet required in this method (but there is others 
  points to enhance in the file) so requirements for this module will be less
  importants.
- add some logs if the command status output is not 0
- with this enhancements, it's not yet necesary to set "Desired Width" AND
  "Desired Height" in the module creation user interface. Just one is required 
  (but it's always possible to set the two's), convert will determine the 
  other dimention using proportion if there is only one.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@18932 20353a03-c40f-0410-a6d1-a30d3c3de9de