Rework Drag-And-Drop API (#4079)

* Add cursor position drag and drop events.
* Reword drag events to match pointer ones.
* appkit: Use `convertPoint_fromView` for coordinate conversion.
* appkit: use ProtocolObject<dyn NSDraggingInfo>.
* x11: store dnd.position as pair of i16

  It's what translate_coords takes anyway, so the extra precision is
  misleading if we're going to cast it to i16 everywhere it's used.

  We can also simplify the "unpacking" from the XdndPosition message--we
  can and should use the value of 16 as the shift instead of
  size_of::<c_short> * 2 or something like that, because the specification
  gives us the constant 16.
* x11: store translated DnD coords.
* x11: don't emit DragLeave without DragEnter.
* windows: only emit DragEnter if valid.
* windows: in DnD, always set pdwEffect.

  It appears other apps (like Chromium) set pdwEffect on Drop too:
  61a391b86b/ui/base/dragdrop/drop_target_win.cc
* docs: make it clearer that drag events are for dragged *files*.
* examples/dnd: handle RedrawRequested event.

Co-authored-by: amrbashir <amr.bashir2015@gmail.com>

src/changelog/unreleased.md

@@ -165,6 +165,25 @@ changelog entry.
 - Rename `VideoModeHandle` to `VideoMode`, now it only stores plain data.
 - Make `Fullscreen::Exclusive` contain `(MonitorHandle, VideoMode)`.
 - On Wayland, no longer send an explicit clearing `Ime::Preedit` just prior to a new `Ime::Preedit`.
+- Reworked the file drag-and-drop API.
+
+  The `WindowEvent::DroppedFile`, `WindowEvent::HoveredFile` and `WindowEvent::HoveredFileCancelled`
+  events have been removed, and replaced with `WindowEvent::DragEntered`, `WindowEvent::DragMoved`,
+  `WindowEvent::DragDropped` and `WindowEvent::DragLeft`.
+
+  The old drag-and-drop events were emitted once per file. This occurred when files were *first*
+  hovered over the window, dropped, or left the window. The new drag-and-drop events are emitted
+  once per set of files dragged, and include a list of all dragged files. They also include the
+  pointer position.
+
+  The rough correspondence is:
+  - `WindowEvent::HoveredFile` -> `WindowEvent::DragEntered`
+  - `WindowEvent::DroppedFile` -> `WindowEvent::DragDropped`
+  - `WindowEvent::HoveredFileCancelled` -> `WindowEvent::DragLeft`
+
+  The `WindowEvent::DragMoved` event is entirely new, and is emitted whenever the pointer moves
+  whilst files are being dragged over the window. It doesn't contain any file paths, just the
+  pointer position.
 
 ### Removed