Draft: Add overrides for Gtk.Accessible update_relation

After discussions with @ebassi, we can likely add Gtk.AccessibleRelationList as a boxed type within GTK 4.5.

Once that is added we can drop the Gtk.AccessibleRelationList override in this MR.

The first two commits don't depend on interface overriding, the last one does.

Depends on !665 (merged)

overrides: Override Gtk.Accessible update_relation

overrides: Add Gtk.AccessibleRelationList

This function wraps a list of Gtk.Accessible objects in a GList and stores the result as a pointer in a GValue.

The pointer is not accessible from JavaScript.

gi: Allow GObject.Value boxed type in all contexts

Previously we only handled this conversion in the arg cache.