Runtime: Add more usages of explicit

Prevents error-prone implicit conversions where applicable.

This targets the regular RuntimeCommon target this time around.

Runtime/MP1/CQuitGameScreen.hpp

@@ -36,7 +36,7 @@ public:
   EQuitAction Update(float dt);
   void Draw();
   void ProcessUserInput(const CFinalInput& input);
-  CQuitGameScreen(EQuitType type);
+  explicit CQuitGameScreen(EQuitType type);
 };
 
 } // namespace MP1