Add wizard for importing Skyline config and config backup

[DominikNovosel]

Since Strato will presumably be released with a different package name and signature, it may not be possible to update the user's Skyline installation automatically. Thus, migrating to Strato would be a pain without some semi-automatic way to import things such as saves, directories, global and per-game settings and controller layouts.

It would also be nice to have the option to export and import configs, either for safekeeping or migrating between Strato installations.