LastGenre: allow prefer_specific without canonical
LastGenre: allow prefer_specific without canonical

This improves lastgenre's behaviour when the configuration option
`prefer_specific` is set but `canonical` is not.
Previously it would not set any tags. Now it does apply tags, sorted using the
canonicalization tree, but not canonicalized.

For this the default tree is loaded even when `canonical` is not set.
An extra check is added to only use it for canonicalization when `canonical` is
set.

beetbox/beets,sampsyo/beets,jackwilsdon/beets,SusannaMaria/beets,ibmibmibm/beets,sampsyo/beets,ibmibmibm/beets,shamangeorge/beets,sampsyo/beets,jackwilsdon/beets,beetbox/beets,ibmibmibm/beets,sampsyo/beets,shamangeorge/beets,SusannaMaria/beets,shamangeorge/beets,SusannaMaria/beets,SusannaMaria/beets,shamangeorge/beets,jackwilsdon/beets,beetbox/beets,beetbox/beets,jackwilsdon/beets,ibmibmibm/beets