Dependency version cycle makes it hard to bump the accesskit version

[valadaptive]

AccessKit v0.18 is out, and I wanted to bump the AccessKit version in egui. However:

• egui_kittest passes AccessKit objects between egui and kittest (this repo), so to bump AccessKit in egui, you must first bump AccessKit in kittest
• But, this repo's basic_integration example depends on egui, so before we can bump AccessKit in this repo, we have to bump it in egui!

Not sure what the best way to solve this is.

[lucasmerlin]

It's quite annoying, but I think it's fine to just update accesskit here first. It'll break the example and CI for while but I think that's preferrable to just not having the examples at all.

[lucasmerlin]

We handled it the same last accesskit update: #3