[Feature] Allow multiple folder/system association and system grouping

[XargonWan]

Allow the mapping of multiple folder as a same IGDB system, for example I may want to have the roms differentiated by system in Commodore:

roms/c64
roms/c128
roms/cmax

In the config file this would become:

c64: c64
c128: c64
cmax: c64

However IGDB is only supporting c64 as system that is actually a grouped system, so I may map my folders to be read all in the IGDB c64 system.

To figure out: Do we want that RomM is merging the three systems or differentiate them as well by checking the folders? Maybe this can be a flag such as the games grouping?

Oppositely this can apply to other systems as nes and fds to unify them into a same virtual system:

nes: fds-fdsfolder:fds
nes: nes

In this case RomM would treat fds-folder as the fds system, but usually the roms will be inside nes system.

Note: nes: fds-fdsfolder:fds is very ugly, better ideas are welcomed.

[zurdi15]

Note: nes: fds-fdsfolder:fds is very ugly, better ideas are welcomed.

Maybe something like this: "nes;fds-fdsfolder": "fds" ?

I need to think the best design for this in any case