Fix rom/folder core selection

[acmeplus]

It's currently possible to select a different core/launcher from the default one for a system or for a rom. In the folder or rom views, pressing SELECT key opens the core override selection (for all folder roms or that particular rom). However, the setting is not preserved/saved.

A proposed solution is to:

• Create a new folder specific view settings json that contains the pairs of folder/core override
• Similarly, create a new item entry for any rom core override in the global_cache.json list
• If a core is overridden per folder, use that instead of the first entry in the folder ini definition (e.g. arcades.ini, NEOGEO entry)
• If a core is overridden per rom, use the core listed for that particular rom as defined in global_cache.json core element