Initial version of libgames-scores

Fake pull request for initial review of libgames-scores.

Hint: scroll down towards the bottom to get to the newer comments.
This won't be translated -- you need to mark the string with N_() if you're going to pass it to _() later.

But it'd be easier to read (and require fewer lines) if you get rid of the extra local variable, and just set the title in each branch:

headerbar.title = _("Congratulations!");