Support storing ROMs in arcade/

[mittonk]

@cmitu suggests in RetroPie/RetroPie-Docs#156 :

One question though - if the games emulated are arcade games, why a separate system ? using arcade as a system is fine and will help with scraping also.

FBNeo lists arcade, fba, and others; MAME lists arcade, mame, and others. Seems like a fine plan.

(DICE supports a small and weird (dmy-file) enough set of ROMs that I like keeping the option of storing them separately from MAME/FBNeo roms.)

[mittonk]

1. Definitely affects RetroPie integration. RetroPie suggests storing arcade ROMs together in arcade but gives the option of separate MAME and FBNeo directories.
2. Need to poke straight RetroArch, systemid and system_name are specified in the dist/info/ file but might not limit ROM locations.
3. Definitely affects future Batocera integration, that defaults to MAME and FBNeo having their own separate directories.

[mittonk]

1. Updated RetroPie-Setup, RetroPie-Extra, Retro-Pie-Docs branches to allow either arcade or dice.
2. I don't see any changes needed for libretro-super/dist/info at least.
3. Future.

[mittonk]

Next question on the RetroPie-Setup side is whether to add the .dmy extension to roms/arcade, discussing in that PR.

[mittonk]

Supporting arcade/ in general, discussing details in appropriate pull requests.