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-wayland/src/window/state.rs

@@ -243,6 +243,12 @@ impl WindowState {
         }
     }
 
+    // HACK: Currently to get the data device to initiate a drag-and-drop, we iterate through all
+    // focused seats to find one with a pointer capability. This is definitely wrong.
+    pub(crate) fn focused_seats(&self) -> impl Iterator<Item = &ObjectId> {
+        self.seat_focus.iter()
+    }
+
     /// Apply closure on the given pointer.
     fn apply_on_pointer<F: FnMut(&ThemedPointer<WinitPointerData>, &WinitPointerData)>(
         &self,