Only listen to targets updates

Thanks @paulkaplan

This reduces the complexity of the reducer, as well as how frequently it's run. The VM will emit a targets update after a step if any target info has changed.

Add Save and Load buttons

/ht @paulkaplan

Integrate VM serialization changes

Allow pointer events to fall through the overlay

Use the de/serialization features of the VM to allow downloading and loading project JSON

It doesn't help anything. Doing a deep compare is unnecessary because the underlying components will do the necessary comparison themselves. Simply seeing if the sprite list has changed at all is enough.

Otherwise it's very annoying to get access to the storage engine to load asset data.

Add reducer, components and containers for monitors

Minor-pin all Scratch 3.0 prerelease modules

Toggle turbo mode when shift clicking the green flag

Pin these modules to 0.*-prerelease.* to avoid problems when we
eventually bump to 1.x

Add prompt for variable creation

Stringify project data

Otherwise the VM can't JSON.load it.

Stop clobbering loading projects with default data

Fix whitespace canvas

Replace draggable with direction input

Get `Asset` and `AssetType` from storage instance

Update babel-loader to the latest version 

This version of babel-loader dropped support for Node < 4. Although 4.2 >= 4, update to 6  for consistency with our other repos, and so that we stay ahead of the curve for any other packages.

If the app is loading with an initial project id, don't try to load the default project. Previously, this data would load on top of the project being loaded.

Does not fix the issue of blocks not loading in the blocks pane, which appears to be separate.