Convert `release_type_scores` option to `ListOption`

We had a space-separated list of name,value tuples ("Album 0.5 Other 0.4"),
and two helper functions to load and save data in the correct format
(`load_release_type_scores` and `save_release_type_scores`).

Since option is converted to recently-introduced ListOption, data is now saved
as a list of tuples, eventually converted to dict when needed.
It simplifies code, and also prevents an issue if it comes we have a release group type
with a space in its name.

An upgrade hook was added to handle this change on Picard upgrade.