Per-system/per-emulator main config overrides

[Miss-Inputs]

Extension of thinking from #143, might make sense to override libretro_frontend for a given core, for example, or wine_path in given scenarios

Or maybe just those? Certain stuff doesn't really make sense to override

And I guess if we're doing that, we would change find_software_by_name and find_software_by_product_code to just be booleans that get overriden per system, instead of string lists of system names