New drag and drop API (#4571)

This commit implements a new API for drag and drop, with a `DataTransfer` type which abstracts over the various clipboard/drag and drop APIs across different platforms. I built this on top of #2429 although admittedly I ended up removing pretty much all of their work while I was reworking the design.

This is being built in order to help support [drag-and-drop work](https://github.com/slint-ui/slint/issues/1967) in Slint's winit backend. As part of that work, I did extensive research on how drag-and-drop and clipboard APIs are implemented across different platforms, and wrote a (still WIP) research document that can be found [here](https://gist.github.com/eira-fransham/06750cf8d25ade08d362a0ca8dfafe06).

The new API is inspired by the browser's [`DataTransfer`](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer) API.

winit-appkit/Cargo.toml

@@ -36,6 +36,8 @@ objc2-app-kit = { workspace = true, features = [
     "NSControl",
     "NSCursor",
     "NSDragging",
+    "NSDraggingItem",
+    "NSDraggingSession",
     "NSEvent",
     "NSGraphics",
     "NSGraphicsContext",
@@ -46,6 +48,7 @@ objc2-app-kit = { workspace = true, features = [
     "NSOpenGLView",
     "NSPanel",
     "NSPasteboard",
+    "NSPasteboardItem",
     "NSResponder",
     "NSRunningApplication",
     "NSScreen",