Scenarios and metadata

Partial rewrite of scenario handling in SP menu

Not even pretending that splitting this into small chunks helps understand any of the changes, simply because it's still horrible.

New bugs introduced due to changed scenario structure: Now require metadata previously stored in root level to be defined in an explicit `metadata` dictionary instead. This prevents parsing entire files multiple times when querying difficulty, author, description and so on.

As for backwards compatibility, we need to re-translate all translated scenarios using to the new scenario translation script instead and just declare old scenarios incompatible. This provides updated versions of everything we previously shipped.

Updates to the English parent scenarios and the translated scenarios are included in this pull request.