Resolve "Update leaderboard screen to use scene2d"

Changes in this merge request

• Use scene2d to create layout for LeaderboardScreen
• Create a setLeaderboard method in LeaderboardScreen that populates the leaderboard or displays an error message
• Add a back button that transitions back to MainMenuScreen
• Update LeaderboardController to set an event listener on the back button

Important ❗

The setLeaderboard method must be called in the onResult method and in the onFailed method in LeaderboardController in order to populate the leaderboard. As of now, the method is not called anywhere.

Closes #32 (closed)