Initial version of libgames-scores

Fake pull request for initial review of libgames-scores.
I guess I should have been more clear about how we declare lambas. It should look like this:

```
private static Gee.HashDataFunc<Category?> category_hash = (a) => {
    return str_hash (a.name);
};
```

Do a search for => throughout your code and fix all the places, please!