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.

typos.toml

@@ -5,6 +5,7 @@ TME_LEAVE = "TME_LEAVE"             # From windows_sys::Win32::UI::Input::Keyboa
 XF86_Calculater = "XF86_Calculater" # From xkbcommon_dl::keysyms::XF86_Calculater
 ptd = "ptd"                         # From windows_sys::Win32::System::Com::FORMATETC { ptd, ..}
 requestor = "requestor"             # From x11_dl::xlib::XSelectionEvent { requestor ..}
+unknwn = "unknwn"                   # Windows SDK header filename `unknwn.h`
 
 [files]
 extend-exclude = ["*.drawio"]