Active progress

@kadst43 @AmbientOSX 

Before this change, the continue scenario would not change while playing the game, so if you started the game with continue pointing to (for example) scenario1h and you got to scenario3a before exiting to the main menu, the continue button would take you back to scenario1h. Now, the scenario you just left is stored as the continue scenario when exiting to the main menu. The "continue" option will take you to that scenario (scenario3a) instead.