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/util/geometry.rs

@@ -95,7 +95,7 @@ impl FrameExtentsHeuristic {
 
 impl XConnection {
     // This is adequate for inner_position
-    pub fn translate_coords(
+    pub fn translate_coords_root(
         &self,
         window: xproto::Window,
         root: xproto::Window,
@@ -103,6 +103,19 @@ impl XConnection {
         self.xcb_connection().translate_coordinates(window, root, 0, 0)?.reply().map_err(Into::into)
     }
 
+    pub fn translate_coords(
+        &self,
+        src_w: xproto::Window,
+        dst_w: xproto::Window,
+        src_x: i16,
+        src_y: i16,
+    ) -> Result<xproto::TranslateCoordinatesReply, X11Error> {
+        self.xcb_connection()
+            .translate_coordinates(src_w, dst_w, src_x, src_y)?
+            .reply()
+            .map_err(Into::into)
+    }
+
     // This is adequate for surface_size
     pub fn get_geometry(
         &self,
@@ -189,7 +202,7 @@ impl XConnection {
         // that, fullscreen windows often aren't nested.
         let (inner_y_rel_root, child) = {
             let coords = self
-                .translate_coords(window, root)
+                .translate_coords_root(window, root)
                 .expect("Failed to translate window coordinates");
             (coords.dst_y, coords.child)
         };