feature #5516 :   Now SilverpeasFileProvider becomes the low-level API to manage files in Silverpeas. It provides currently methods to write, to delete and to move a file. This new API  is yet used by ImageProfil (it manages the avatar) and ThumbnailController (it manages the vignettes and thumbnails of some contributions like the publications). ImageResizingProcessor that is hooked to SilverpeasFileProvider takes advantage of this to manages the cache of resized images when their original image is either deleted or updated. Beside that, a ResizedImageCleaner is introduced to clean up resized images of non existing original images; it is scheduled by a cron instruction in the org.silverpeas.lookAndFeel.generalLook.properties bundle.