Improve Atari 7800 support

[sehugg]

• Missing modes: 320C, 320D, 160A, and 160B.
• Bank switching
• More examples
• Is there a test suite / test cart?
• More debugging -- add display list to debug tree?
• Two-button controllers
• No difficulty switch?

[sehugg]

Can you add in some bankswitching modes for 7800? Specifically these 3-

1. POKEY @ 800 - this isn't a bankswitching mode lol, but POKEY is something you can add to a 7800 cart so I think it should be supported.
2. RAM @ 4000 - 7FFF
3. 128K cartridges- Think of this as a giant $00000-$1FFFF space. $1FC00-$1FFFF is mapped to $C000-$FFFF. $8000-$BFFF can be banked by writing a number from 0 to 7 to anywhere from $8000-$BFFF. Ignore the top 5 bits.

Even better would be if you could support 512K or Bankset (seperate MARIA graphics)