Drag-and-drop: originate in Kivy, drop in some external app

[janssen]

You should be able to initiate a drag from a Kivy widget, which would put a string or an image or a filename into the native drag-and-drop framework, to be dropped on some external application. Drag stuff out of Kivy.

[kiok46]

@janssen This issue should be opened in kivy or maybe a flower in kivy-garden?
But as plyer is independent of kivy, we can not(should not) design such functionality only for it.`

[dessant]

Duplicate of kivy/kivy#3557.