DolphinQt/Config/ARCodeWidget: Use forward declarations where applicable

Avoids propagating headers into scopes where they're not necessary.

Also uncovered reliance on an indirect inclusion within
CheatsManager.cpp, which is now fixed.

Source/Core/DolphinQt/Config/ARCodeWidget.h

@@ -10,7 +10,11 @@
 #include <vector>
 
 #include "Common/CommonTypes.h"
-#include "Core/ActionReplay.h"
+
+namespace ActionReplay
+{
+struct ARCode;
+}
 
 namespace UICommon
 {
@@ -28,6 +32,7 @@ class ARCodeWidget : public QWidget
   Q_OBJECT
 public:
   explicit ARCodeWidget(const UICommon::GameFile& game, bool restart_required = true);
+  ~ARCodeWidget() override;
 
   void AddCode(ActionReplay::ARCode code);