Removes all remaining usages of the global system instance. After this,
migration can begin to migrate to being constructed and managed entirely
by the various frontends.
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.
This adds two options to the General -> UI tab. The first disables picking a place to save the file. The second chooses a default directory for saving screenshots.
