Keyboard support for SG-1000/Mark III

[Tastyfish]

I couldn't find a game that used more than 10 keys or so, since most just use the arrow keys, Insert, and Home, basically pretending they make up a controller, so I can't guarantee that all the keys actually work.

SG-1000 games tested:

• Girl's Garden
• Sega-Galaga
• Champion Boxing
  • Actually uses more keys if you select 2 player!

Mark III Games tested:

• F-16 Fighting Falcon
  • I barely have any clue what's happening in this game, but I managed to shoot at a plane so I assume it all works.
• Alex Kidd no Miracle World*
  • This one is weird, it detects the keyboard, and you can start the game with it, but once it launches into the first level, it zero-outs the flags byte that includes the detected controllers and just forcibly uses the basic controller in-game.

Resolves #1059

[alyosha-tas]

Awesome!
Tested Champion Tennis (which originally prompted this request) and it works fine.

[Tastyfish]

Thanks, I wasn't sure which game it was that prompted it.