Update to AccessKit 0.12

[mwcampbell]

The biggest change in glazier itself is that glazier, rather than the application, is now responsible for telling AccessKit about the focus state of the window, because this is now tracked in the AccessKit platform adapter rather than in the tree itself. Most of the other breaking API changes affected the example.

[DJMcNab]

Apologies, didn't realise that this PR had been opened.

These changes look good, thanks. The one surprise is that the focused bool in the accesskit adapters is per-window. Will there be any change needed there to allow multi-window applications, or does each window naturally get its own adapter (i.e. there's no risk of them overwriting each other)

I think you should have permission to merge this yourself, but if not let me know

[mwcampbell]

Each window gets its own adapter. We even decided to make the API work that way for our AT-SPI backend, at the cost of some implementation complexity, both for consistency with other platforms and because our ideal future GNOME accessibility stack will not have a top-level application object as AT-SPI does.