Add drag-and-drop APIs with payloads storage

[emilk]

• Closes Improvements to make drag-and-drop easier #3882

This adds several methods to make drag-and-drop more ergonomic in egui.

In particular, egui can now keep track of what is being dragged for you (the payload).

Low-level:

• egui::DragAndDrop hold the payload during a drag

Mid-level:

• Response::dnd_set_drag_payload sets it for drag-sources
• Response::dnd_hover_payload and Response::dnd_release_payload reads it for drop-targets

High-level:

• ui.dnd_drag_source: make a widget draggable
• ui.dnd_drop_zone: a container where things can be dropped

The drag-and-drop demo is now a lot simpler:

https://github.com/emilk/egui/blob/emilk/drag-and-drop/crates/egui_demo_lib/src/demo/drag_and_drop.rs

[abey79]

I'm only able to use the low-level API in rerun, because of all the geometry testing I must run on my side, but the ability to store and retrieve the dnd payload (along with using its existence as a marker of on on-going drag) makes the code much cleaner and easy to reason with.

[emilk]

@abey79 Oh, you can't even use the Response API? :(

[abey79]

@emilk The response API works well when moving boxes inside boxes, but not so well when moving boxes in-between boxes :)