lastgenre: Allow keeping of non-whitelisted genres instead of deleting them.

[Nukesor]

When no genres can be found or no genres match the canonical whitelist, the current logic either sets the fix value of fallback or deletes the existing genres.

Removing genre info and/or setting them to an arbitrary values makes it both tricky to adjust canonicalization settings (as you don't see what's going on), and I would rather like to keep non-whitelisted genre info than simply delete it.

Proposed solution

A fallback_keep_existing toggle config flag.

Goals

Allow users to keep genres that cannot be cannonicalized, instead of wiping the genre tags.

Anti-goals

The logic is very isolated, as far as I can see there shouldn't be any issues. As a matter of fact, this operation is even less-destructive, so even in case of a bug, it would result in less deleted data.

[Nukesor]

I already started work on this, but my branch requires #6305 to be merged.

The commit can already be reviewed over here:
Nukesor@2642ea8