Add incremental add and update to beets catalogs

With this Pullrequest we are finally able to do quick adds and updates on the library.
When adding to the library, only the new added songs since the last add get fetched from beets.

When verifying, it is the same. But be aware that we only get changes when beets knows the changes. So maybe you need to do a "beet update [query]" or similar before.

We cannot detect path changes yet, so a "beet mv [query]" will probably lead to new songs in the ampache library. But I will do this as soon as I figure out how to add custom properties / metadata to ampache.