And use instanceOf for the vm prop type

Move behavior-related components to "containers", leave presentation components in "components".

Confine the logic for adding/removing event listeners between the VM and the components to the relevant components.  While this tightens the coupling between the component and the environment, it makes using the individual components more self-contained.

Update webpack config and vm usage to match updates in scratch-vm

Also make the toolbox actually get used by scratch-blocks, and update the import of scratch-blocks to use the new shimming method.

This way we can have a consistent style for Node and web code.

Add skeleton toolbox component

Thanks to babel-loader and babel-eslint, we can use the same linting everywhere.  This makes our imports easier since we don't need to think about the extension.

Also export things more succinctly in src/index.js