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>

examples/window.rs

@@ -541,11 +541,12 @@ impl ApplicationHandler for Application {
                 info!("Smart zoom");
             },
             WindowEvent::TouchpadPressure { .. }
-            | WindowEvent::HoveredFileCancelled
+            | WindowEvent::DragLeft { .. }
             | WindowEvent::KeyboardInput { .. }
             | WindowEvent::PointerEntered { .. }
-            | WindowEvent::DroppedFile(_)
-            | WindowEvent::HoveredFile(_)
+            | WindowEvent::DragEntered { .. }
+            | WindowEvent::DragMoved { .. }
+            | WindowEvent::DragDropped { .. }
             | WindowEvent::Destroyed
             | WindowEvent::Moved(_) => (),
         }