Trait upcasting (rust-lang/rust#65991) is stable since Rust 1.86, which
is already the MSRV, so dyn trait objects coerce to dyn Any directly.
Traits that needed AsAny now list Any as their supertrait, and the
as_any module is renamed to casting as only the impl_dyn_casting macro
remains.
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.
