Auto adding: Option to auto add files of all types, not just ebooks

Auto adding: Option to add files of all types, not just ebooks. You can
now tell calibre to auto add all file types, instead of just known
ebooks via Preferences->Adding books->Automatic adding. Fixes #1252814 [[Enhancement] Automatic Add functionality for all file types](https://bugs.launchpad.net/calibre/+bug/1252814)