Better implement some parts into their separate functions.

Define a new window for settings.

Add indicators, hide controls on mouseout, and put pedal board in
bottom.

For consistent look and feel.

Bootstrap to later Electron version.

Add custom themes and colors for keys.

Use updated version of `react-musical-keyboard`.

Use custom button for closing, improve app window.

Still not working, but updated README nevertheless.

Use create-react-app to use first-party `react-musical-keyboard` component.