I'm developing a theme and working on the Customizer at the moment. I've been able to add sections, panels and controls to change text, colours and images.
One thing I've seen on other themes is an "Add A Widget" button which allows you to add widgets into the panel. There's an image below of the button in the Responsive Boat theme.
I'd like to add this into the Customizer in my theme but can't find any information on how to do it. Does anyone know how to do this or have any links to information about it?