Fixed multithread download, minor changes on download management

[EnricoTirri]

Changes includes:

• Migrated progress listener functionality from Video to AnimeDownload
• Fixed and change multithread ("multistream") functionality
  • Safe mode introduced (single stream download with no range request, always re-download the whole file)
  • Requests are always intended multistream (single stream is only a particular case)
  • Download stop/pause/resume now works, fixed job cancellation problems
  • On not-safe-mode download caches partial downloaded file, stop and resume will continue taking account of it
  • Fixed download progress computing error (that was brutally controlled by hot fixes)
  • Slightly modified download retrying routine (see more in the code comments)
  • Modified error handling, less try catch routines and now errors should appear in the right way into notifications

[giorgionegro]

Closes #1325