Implement drag files from kupfer's window

[seltzered]

On similar quazi-modal launcher's like quicksilver, a user can drag a file brought up from the launcher window and drop it somewhere else. As someone that's used quicksilver for a decade, this usecase is extremely useful for instances where one wants to say, drop a screenshot into an attachment, drop a file into a gnome-files window that's already open, etc.

Environment

Ubuntu 21.04 (wayland)

Actual Behaviour

Dragging a file or folder from kupfer doesn't appear to have any effect.

Expected Behaviour

Dragging a file or folder from kupfer should be possible and show appropriate visual cues similar to dragging and dropping a file when using a file manager.