Add load options for PRG and CHR banks

[CBongo]

The primary feature of this commit is to add load options for PRG and CHR banks. Selecting the Options button in the load dialog will show (if appropriate for the mapper) choices for the base address of each PRG bank, and a choice of how to handle CHR banks.

There is a fair bit of refactoring going here to support this, as well as preparing for more generalized mapper support in the future. In particular, there has been some shuffling of code between the NesRom, GhidraNesLoader, and NesMapper classes to better align them with their logical responsibilities. The separate classes for UxROM and MMC4 have been rendered moot as a result, so they have been removed.

I've also started adding more informational messages that will appear in the log window when loading a ROM.

Oh, and just for fun, I also updated the README.md with instructions on how to adjust fallthrough when mappers swap banks out from under you. I didn't have a chance to add screenshots yet, though.

@kylewlacy As this is a bit more significant of a change, I was hoping you'd chime in if you have thoughts on whether this approach seems reasonable.

[kylewlacy]

LGTM!

Awesome to see that several custom classes just fully deleted while still adding more mappers! To me that's the clearest sign that this is the right direction to go. Love to see the extra debug output too

(Feel free to merge whenever, I only held off because I saw some TODO comments so I wasn't sure if you were planning on pushing more changes before merging or if those were for future follow-ups)