Request: Winit 0.31 support

[nicoburns]

Winit 0.31 beta has been released, and AccessKit (specifically accesskit-winit will need to be updated to support it).

Some notes:

• Winit 0.31 removes the generic UserEvent type from the EventLoopProxy. It's replacement is a simple wake_up function. The idea is that you would send your actual message through something like a channel (std::mpsc, or similar).
• Winit now has a type-and-trait only winit-core crate.

I suspect that:

• AccessKit should be able to depend on just winit-core (it should not need to depend on winit)
• Rather than trying to directly send events to the Winit EventLoopProxy, it should probably just accept a callback and let the user choose what they do with the events.

(I may have some time to work on this)