Fix unplayable game when changing difficulty

Previously, changing the difficulty before the first game was started
by selecting "Single Game" or "Human vs. AI" started an unplayable
game.

Now, levelChanged() calls singleGame() or pveStepGame() instead of
startGame(). These functions are also called when selecting "Single
Game" or "Human vs. AI", respectively, and correctly connect keyboard
events to the game.

BUG: 339038
REVIEW: 127171