Improvements to make drag-and-drop easier

- #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()`