FlappyBug: Keep track of score and highscore

Better information is now shown to the player. Instructions are shown
when first loading the program, and any available scores are shown on
the game over screen.

Userland/Games/FlappyBug/Game.h

@@ -33,6 +33,7 @@ private:
 
     void tick();
     void reset();
+    void game_over();
 
     struct Bug {
         const float x { 50 };
@@ -95,6 +96,8 @@ private:
     Bug m_bug;
     Obstacle m_obstacle;
     bool m_active;
+    Optional<float> m_highscore;
+    float m_last_score;
     float m_difficulty;
 };