DnD: support drop callbacks

This issue covers the quantum/glass part of RT-14733 and is related to supporting promised files (RT-21981), but is more general - the drag source could issue just a callback to get the data (instead of putting the data on dragboard) and the drop target would then use this callback to retrieve the data after drop.

I'm not sure on which platforms this is even possible, the native support for such thing needs to be evaluated and then we can decide whether and what to provide. This is not a high-priority issue, but it would be good to keep it in mind and do the evaluation on all platforms when working on the promised files (RT-21981).