Make GtkAccessible a public interface

This MR makes GtkAccessible public so it can be implemented from other libraries (Libadwaita definitely needs it to fix AdwEntryRow's a11y). It also decouples the widget tree almost completely from the accessibility tree, so, for example, the GtkStackPage a11y objects can be defined in GtkStackPage, and not in the backend. Note that I would like to remove all notions of GtkWidget and friends from the backend code, however I am not sure whether I can get rit of the widget_is_realized calls. To add to that, the top-level widgets would have to be racked somewhere else, so these two thinks still access the widget APIs.