Updating and staying up-to-date with latest changes

[Baklap4]

Sometimes songs get updated in USDB. This can be done through adding a comment, updating the txt file and maybe at a later point correctifying through usdb-syncer (#111)

As a user of USDB-Syncer i'd love to be told that a song i have previously downloaded is out of date and needs to be updated.

Currently to achieve the same behavior you'd have to select all the songs you've downloaded and add them to a batch to download again. Sure a lot will be skipped, but those checks need to happen for every song.. this is a major performance concern and not needed if implemented correctly.

One way to deal with this problem is:

• [ ] Add OutOfDate to the localSongs object
• [ ] Determine when a song is out of date
  • [ ] Fetch songs and check when it's last been edited/updated
  • [ ] Check the comments for updates
  • [ ] Execute once (during startup) or on interval (2 hours) to keep the song-list up-to-date
• [ ] Add OutOfDate to the song-list and batch download