Update VM to support importing monitors

Add "read-only" slider and list monitors.

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

Consume monitor mode and position from VM

Add setAppElement to GUI.

Fixes #1853 

Add random backdrop menu option

Fixes #1853

Update scratch-render to the latest version 

Replace buffer-loader with arraybuffer-loader

Replace babel-preset-es2015 with babel-preset-env

Optimize the logo SVG

Update to webpack 4

It was 435KB. Now it's 5KB. I used SVGOMG: https://jakearchibald.github.io/svgomg/

Add several new integration tests to the GUI

Large readout mode for monitors

Root cause is probably the flex implementation on Safari.

Sprite upload