Sneak peek of the Drag And Drop sample ( <https://...

# gui

[napperley]

Sneak peek of the Drag And Drop sample ( https://gitlab.com/gui-vista/samples/dnd-sample ), which is a WIP and is a translation of the Drag And Drop Python example ( https://python-gtk-3-tutorial.readthedocs.io/en/latest/drag_and_drop.html ) with some differences 🎉.