Add in-game leaderboard display

[peppy]

We have a pretty nice leaderboard display in multiplayer, but it's currently missing in both solo and playlist modes.

The leaderboard implementation will likely need some server-side considerations, as we want to (hopefully) display more than the top 50 players this time around.

A good starting point would get the leaderboard working with local scores. Online scores can be added as a subsequent step.