Allow multiple <path> instances on system entries

[muzzol]

this is a feature request to allow multiple path definitions on system entries.

with the rise of handheld devices like steam deck and such, is very convenient to have roms in multiple locations, even for the same system (internal drive, microsd, external usb drive).

this is specially usefull for platforms with huge roms that use a lot of space (wii, PS3).