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.
By using `OwnedDisplayHandle`, which uses reference-counting, instead of
`DisplayHandle<'static>` (where the user has to guarantee that the
Application doesn't outlive the event loop).
Unify the values of `MouseButton` and thus remove `Other` variant in
limit possible buttons to 32, which was picked based on platform
capabilities, where 32 is the highest.
For the reference, SDL has identical limit.
When pressing Ctrl+Space, KeyEvent::text_with_all_modifiers would return
\0. That would get included in the text-input text. When an input method
gets activated, the invalid string returning \0 would get sent as
surrounding text, resulting in a Wayland protocol error and crashing the
application.
