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/dnd.rs

@@ -4,6 +4,7 @@ use std::path::{Path, PathBuf};
 use std::str::Utf8Error;
 use std::sync::Arc;
 
+use dpi::PhysicalPosition;
 use percent_encoding::percent_decode;
 use x11rb::protocol::xproto::{self, ConnectionExt};
 
@@ -45,13 +46,25 @@ pub struct Dnd {
     pub type_list: Option<Vec<xproto::Atom>>,
     // Populated by XdndPosition event handler
     pub source_window: Option<xproto::Window>,
+    // Populated by XdndPosition event handler
+    pub position: PhysicalPosition<f64>,
     // Populated by SelectionNotify event handler (triggered by XdndPosition event handler)
     pub result: Option<Result<Vec<PathBuf>, DndDataParseError>>,
+    // Populated by SelectionNotify event handler (triggered by XdndPosition event handler)
+    pub dragging: bool,
 }
 
 impl Dnd {
     pub fn new(xconn: Arc<XConnection>) -> Result<Self, X11Error> {
-        Ok(Dnd { xconn, version: None, type_list: None, source_window: None, result: None })
+        Ok(Dnd {
+            xconn,
+            version: None,
+            type_list: None,
+            source_window: None,
+            position: PhysicalPosition::default(),
+            result: None,
+            dragging: false,
+        })
     }
 
     pub fn reset(&mut self) {
@@ -59,6 +72,7 @@ impl Dnd {
         self.type_list = None;
         self.source_window = None;
         self.result = None;
+        self.dragging = false;
     }
 
     pub unsafe fn send_status(