There will probably need to be more backpack config options in the future, so just make this a bit more future proof

Move guiMiddleware to `reducers/gui.js`, and use it in `app-state-hoc`

scratch-gui exports `index.js` instead of just GUI container.

Since compatibility-testing got switched to use the player mode, it doesn’t need any of the stage/controls components.

* Export initial states for each reducer
* combine all gui reducers (excludes scratchPaint and intl)
* combine all initial states to create default guiState for preloading in www
* expect all of the gui state to be accessible at `state.scratchGui`

GUI will export a reducer to be included in the enclosing component’s redux when GUI is used as a library. The app-state-hoc is moved into the playground examples to provide a store when GUI is being used stand-alone.