Support FDS (Famicom Disk System) files for NES

[puzzud]

At least as of Nestopia v1.15.1, FDS (Famicom Disk System) files are supported. However, Retro Debugger does not recognize when I attempt to load one.

Conventional emulators like Nestopia and FCEUX require supplying the FDS BIOS ROM that is expected to be named disksys.rom. So Settings->NES->Select NES ROMs folder will be necessary. As well as the instructions in this repo's README.md to supply the aforementioned ROM file.

Being the disk system introduces a disk drive, menu options to Open, Detach, and swap disks for the NES will be necessary. Drive related utilities would also be nice but obviously not required and should be addressed in a separate issue.