Another update with lots of small changes that move everything closer to being a fully usable system

•   Added Screenshot key (F11)
•   Added beginnings of UI library, and UI test program to demonstrate (lots of buttons!)
•   Added SmallText library, and beginnings of an in-app text editor
•   Paint app shows current attributes under the cursor
•   Added start of "Chess Invaders" example game
•   Fixed lots of minor issues