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/platform_impl/linux/x11/window.rs

@@ -1520,7 +1520,7 @@ impl UnownedWindow {
         // This should be okay to unwrap since the only error XTranslateCoordinates can return
         // is BadWindow, and if the window handle is bad we have bigger problems.
         self.xconn
-            .translate_coords(self.xwindow, self.root)
+            .translate_coords_root(self.xwindow, self.root)
             .map(|coords| (coords.dst_x.into(), coords.dst_y.into()))
             .unwrap()
     }