Background thread: Scan files without lookup

When Picard is used for calculating-submitting AcoustID:s to known MB releases, the automatic match default is a nuisance causing extra work instead of helping. Typically for recording compilations, matches are spread out on any releases other than the one just added, and you have to manually drag files 1 by 1 to the release already known to be correct. It would probably be code-wise impractical to allow for scanning after manual match, but it seems scanning is a 2-fold action (lookup & match), and presumably easy to do an opt-out of the match part of it.

Related: PICARD-134: Picard should match tracks on right pane before looking for them any where else