refactor!: Drop Tree::app_name

[mwcampbell]

I now believe this property was a mistake.

On platforms where assistive technologies have conditional behavior based on the app name, they should get the app name from the platform, not the app itself. This way, correct behavior isn't dependent on the app or toolkit exposing the app name, and we don't give the app the opportunity to lie about its name.

Yes, AT-SPI requires the app to provide its own app name. I consider that a flaw in AT-SPI, which we should work around in the Unix adapter, rather than perpetuating it in our new cross-platform API. In Newton (the new free desktop accessibility stack I prototyped earlier this year), I want ATs to get the app name from the compositor, not the app itself, though I didn't implement that yet.

While I was here, I changed the toolkit_name and toolkit_version accessors in accesskit_consumer to return Option<&str> rather than cloning the Option<String>, since the latter is an unnecessary pessimization.