(DS-specific) Add an option to put the game on the Lower Screen. (GitHub issue #12)

* Change the GBA Guru Meditation screen and the FPS display so that they appear on the configured game screen.
* Before entering the menu, copy the game screen to the Upper Screen if it's on the Lower Screen.
* Prevent the FPS display from appearing if it needs more data to calculate the last second's worth of FPS:
--> when initialising the emulator;
--> when loading a game;
--> while in the menu, the FPS display is dropped completely;
--> for 1 second after exiting the menu.