winit integration

[mwcampbell]

Some popular Rust GUI toolkits and game engines use winit, so we need an easy way for winit-based applications to use AccessKit.

On Windows, the issue is that winit doesn't currently provide an official way for users to handle custom window messages such as WM_GETOBJECT. Possible fixes for this have been discussed on rust-windowing/winit#2120, but nothing has yet been implemented. That's not a complaint; I'm sure everyone working on winit is busy with other things. But I need to implement a solution soon. So I'm going to use a Win32 API technique called subclassing to override the window procedure with our own message handler. I'll implement this in the accesskit_windows crate, then I can add an accesskit_winit crate.

[mwcampbell]

I implemented Win32 subclassing in #118.

[mwcampbell]

My new winit adapter is in the winit branch under platforms/winit. I'll submit a PR once #120 is reviewed.

[maroider]

The use of a T: From<ActionRequestEvent> bound on the ActionHandler impl for WinitActionHandler<T> honestly surprised me (in a good way, of course). The API quite reasonable to me, given the current state of Winit.

[mwcampbell]

I just published the accesskit_winit crate on crates.io. So I'll close this, and open separate issues for any problems that arise in real-world usage.