Add DroppedFile support for Wayland

[chrisduerr]

The DroppedFile event should be supported on Wayland through the data device, but afaik at this point the feature isn't implemented on Wayland yet.

I suppose the HoveredFile and HoveredFileCancelled could also be implemented, but I'm mainly concerned with the DroppedFile event.

[kchibisov]

It uses the same protocol and shares objects as clipboard, so winit should also support clipboard to have drag and drop implemented, which is not the case right now...

[WhyNotHugo]

so winit should also support clipboard to have drag and drop implemented

Not really. They re-use the same objects because copy-pasting and drag-dropping share a lot of the same concepts, but you can implement the latter without the former.

[chrisduerr]

you can implement the latter without the former.

Can and should aren't necessarily the same here though.

[kchibisov]

Duplicate of #1881.