Pass GMenu2X instance by reference instead of pointer

This makes it explicit that null is not accepted.

src/menusettingmultistring.h

@@ -42,7 +42,7 @@ private:
 
 public:
 	MenuSettingMultiString(
-			GMenu2X *gmenu2x, Touchscreen &ts,
+			GMenu2X& gmenu2x, Touchscreen &ts,
 			const std::string &name, const std::string &description,
 			std::string *value, const std::vector<std::string> *choices);
 	virtual ~MenuSettingMultiString() {};