Make slider and list monitors update variables in the VM

Update scratch-vm dependency

This fixes the music manifest issue.

Send svg renderer to vm

Update vm and blocks for block visibility

Fix uint16 sound loading error

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