Add comment about refs to functional components

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>

src/components/views/elements/ImageView.tsx

@@ -83,6 +83,7 @@ export default class ImageView extends React.Component<IProps, IState> {
         };
     }
 
+    // XXX: Refs to functional components
     private contextMenuButton = createRef<any>();
     private focusLock = createRef<any>();