Expose AppStateHOC component

For use from `scratch-desktop` and maybe other places...

src/index.js

 import GUI from './containers/gui.jsx';
+import AppStateHOC from './lib/app-state-hoc.jsx';
 import GuiReducer, {guiInitialState, guiMiddleware, initFullScreen, initPlayer} from './reducers/gui';
 import LocalesReducer, {localesInitialState, initLocale} from './reducers/locales';
 import {ScratchPaintReducer} from 'scratch-paint';
@@ -13,6 +14,7 @@ const guiReducers = {
 
 export {
     GUI as default,
+    AppStateHOC,
     setAppElement,
     guiReducers,
     guiInitialState,