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

* 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.

Specifically in 2.0:
- Monitors cannot be dragged in fullscreen/player
- List monitors cannot be resized, have items added using the button or have their values edited.
- Slider monitors still work.

bowser correctly detects Chrome Headless as Chrome, is already used by scratchr2, and is more actively maintained than platform.

Includes:
- context menu for switching to sliders, only on variables.
- width, height, min, max properties consumed from VM.

Fixes #1853

Update calls to addSprite2 to use new API for adding sprites. Cleanup sprite3 representation for uploading a new sprite from an image.

Also extract the "supportedBrowser" logic into a helper (rule of 3)