Independent song validation

On saving an edited song, there are a few validations (title, author, tags, ...). It should be possible to validate imported songs in the same way.

Idea:
A independent song validator which can be used to validate all songs in the database. Only validated songs can be used in services and in the preview. Not validated songs can only be edited.
Therefor we need a flag in the database where is stored if the song is validated or not. The validation status should be shown in the media item.

When will this validation be called?
1. When saving an edited song (status quo)
2. After every import (with prior user query)
3. Manually in the media item (one song / all not validated songs / all songs)

If a validation fails, the user will be notified on a dialog, which song(s) couldn't be validated and which test failed.