Currently Picard defaults to "." for the move files to destination directory.

This has some implications as discussed in PICARD-638 and is actually a rather advanced setup only recommended if one really wants to have this.

The proposal here would be to:

1. Change the default to be the QStandardPaths.MusicLocation directory. On Linux this is defined by XDG Base Directory config, often ~/Music. On Windows it is the users "My Music" folder.
2. Still allow relative directories including "." here so that users who want to use this can still do
3. Do not migrate this setting. We can't assume just because someone has set it to the default "."

I think this makes the default more useful for more users. It also serves as a better example. If new users see in options the location is e.g. "C:\Users\Laura\My Music" they understand what is meant, if they just see "." they might not understand this setting at all.