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.

Cargo.toml

@@ -36,6 +36,7 @@ rwh_06 = { package = "raw-window-handle", version = "0.6", features = ["std"] }
 serde = { version = "1", features = ["serde_derive"] }
 smol_str = "0.3"
 tracing = { version = "0.1.40", default-features = false }
+url = "2"
 
 # Dev dependencies.
 image = { version = "0.25.0", default-features = false }