This tab of the settings is already extremely bloated and the setting itself is quite useless.
With a gamelist of almost 30 games, the cache directory is smaller than 1MB for me and therefore I don't see why it needs to be configurable.
Migrates a remaining common file over to the Common namespace, making it
consistent with the rest of common files.
This also allows for high-traffic FS related code to alias the
filesystem function namespace as
namespace FS = Common::FS;
for more concise typing.
