Rework the accessibility stack for GTK4

Hello,

commit feb8975a removed the key snooper API, which the accessibility layer was depending on to get keyboard events. The net result is that the screen reader can not get the keyboard event, thus not being able to tell them to the user and breaking a lot of scripts.

Would it be ok to not reintroduce the snooper API, but at least reinstall

case GDK_KEY_PRESS:
case GDK_KEY_RELEASE:
  if (_gtk_accessibility_key_snooper (grab_widget, (GdkEventKey *) event))
    break;

to fix back accessibility?