Improvements to make drag-and-drop easier

[abey79]

• Add some drag-and-drop-related APIs in Response and Memory #3876 added men.dragged_id(), but there is no way to get the value of to an egui::Id (which could then be converted to an internal Id).
• It would be nice to have ctx().anything_being_decidedly_dragged() (equivalent to ui.memory(|mem| mem.is_anything_being_dragged()) && ui.input(|i| i.pointer.is_decidedly_dragging())
• It would be nice to have ctx().drag_stopped()

[emilk]

We should take a look at the Dear ImGui API for drag-and-drop

[crumblingstatue]

I want to mention egui_dnd just as a baseline to compare an egui-builtin solution against.

It only seems to support fairly simple layouts.

I would like something that supports drag and drop between different uis for maximum flexibility.

An example use case of mine would be to drag tree nodes in a tree.

For example move a-1 to b in this tree ui made out of CollapsingHeaders.

[abey79]

@crumblingstatue I think a proper high-level drag-and-drop API in egui is still a little far off due to the difficulty to properly address the general case(s). This issue is more about the little things that would make lower-level implementation easier.

As for the tree drag-and-drop, I just built a demo of that in the rerun repository: rerun-io/rerun#4879. I plan on making a proper egui example out of it.

[abey79]

@crumblingstatue #3903