Reworked AppPreferences update process

Updating the AppPreferences no longer requires increasing a version number, only updating the list of feed URLs does.
Solved problems related to FeedImages

- FeedImages of FeedItems were not deleted when the Feed was deleted
- FeedImages of FeedItems were downloaded multiple times
Merge branch 'master' of github.com:danieloeh/AntennaPodSP
do not download automatically