Use config files to store whether SDMC is enabled or not

Before, it used to use whether the directory actually existed. As a result, .citra-emu/sdmc was never auto-created (something quite confusing to me until I read through the logs).

src/citra_qt/config.h

@@ -14,6 +14,9 @@ class Config {
 
     void ReadControls();
     void SaveControls();
+
+    void ReadData();
+    void SaveData();
 public:
     Config();
     ~Config();