a11y: Align to gtk breaking change in AccessibleValue errormessage relation

[felinira]

We recently had to break public API of GtkAccessibleValue in gtk because it was ambiguous, and not aligned to the W3C-ARIA spec.

https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/7528

We still use this API in the previously documented way from gtk-rs that caused a runtime crash in gtk until now. It should be changed to use a list relation, not a single reference. This should be possible to do in a point release, because right now it just reliably crashes the app whenever it's used.

Opening this issue so that I don't forget about this, I'll fix it myself.