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Links:
- https://www.theodo.fr/blog/2017/04/adding-drag-drop-to-your-web-app-the-simple-way-demos/
- https://shopify.github.io/draggable/ : a drag&drop library
- https://developer.mozilla.org/en-US/docs/Web/API/HTML_Drag_and_Drop_API/Drag_operations#droptargets
- https://www.html5rocks.com/en/tutorials/dnd/basics/
- http://apress.jensimmons.com/v5/pro-html5-programming/ch9.html