Add accesskit_adapter crate. Upgrade accesskit_winit to winit 0.31.0-beta.2

[nicoburns]

• Fixes Request: Winit 0.31 support #654

Motivation

• Upgrade AccessKit's Winit support for Winit 0.31 (currently in beta)
• Create a cross-platform AccessKit adapter that can be used without Winit
• The non-winit cross-platform adapter would also be useful for using AccessKit with new versions of Winit before Accesskit adds support.

Changes Made

• Create a new accesskit_adapter crate that abstract accross the platform adapters without depending on Winit types
• Make accesskit_winit a thin wrapper around accesskit_adapter
• Update accesskit_winit to Winit 0.31.0-beta.2. Notably Winit 0.31 no longer has a generic user event type. The expectation is that users of Winit will send their own events via a side channel such as a std::mpsc channel. I have therefore updated accesskit_winit to accept a generic callback.