Added saving - MAJOR -

Now saves all the images you got correct to a file and then reads that
file back when you start the activity, letting you know if you've
already completed that one.

Added a way to draw a graphic to the screen if you're correct - simply
change correct.png to whatever graphic is suitable and then change where
it's drawn.

Some minor cosmetic stuff:
- Made app fullscreen
- Gave the buttons some transparency, looks nice.