Feat: Smartplaylist: Add dest_regen option for regenerating destination path

[pierreay]

Problem

1. When items are imported into the library in a don't copy-move mode (-C -M options), they will be registered inside the Beets database using their original paths. However, during subsequent processing (e.g., a convert operation), a path following the Beets path format can be generated.
2. When generating playlists using smartplaylist plugin, only the path registered inside the Beets database (the original path) can be use inside the output playlist. This block the compatibility with other plugins.

Solution

I added a a new optional configuration option known as dest_regen (as well as its equivalent dest-regen on the CLI) to regenerate items' path in the generated playlist instead of using the ones of the library, just like a convert or a move operation would have done. This operation will happen before the relative_to and prefix options, which makes sense to do so and not in another order, otherwise this new option (dest_regen)) would overwrite the desired behavior of the other mentioned options (relative_to and prefix). It is then helpful to generate playlists compatible with the convert plugin.

To Do

• Documentation
• Changelog
• Tests. (I may write one in the future if this PR is considered)