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/Cargo.toml

@@ -22,6 +22,7 @@ serde = ["dep:serde", "bitflags/serde", "smol_str/serde", "dpi/serde"]
 bitflags.workspace = true
 cursor-icon.workspace = true
 dpi.workspace = true
+percent-encoding.workspace = true
 rwh_06.workspace = true
 serde = { workspace = true, optional = true }
 smol_str.workspace = true
@@ -40,7 +41,7 @@ sctk = { package = "smithay-client-toolkit", version = "0.20.0", default-feature
 sctk-adwaita = { version = "0.11.0", default-features = false, optional = true }
 wayland-backend = { version = "0.3.10", default-features = false, features = ["client_system"] }
 wayland-client = "0.31.10"
-wayland-protocols = { version = "0.32.11", features = ["staging", "unstable"] }
+wayland-protocols = { version = "0.32.12", features = ["staging", "unstable"] }
 wayland-protocols-plasma = { version = "0.3.8", features = ["client"] }
 winit-common = { workspace = true, features = ["xkb", "wayland"] }