a11y: Handle relations in UI files

a11y: Handle relations in UI files

A bit hacky: we skip parsing values that have a reference or
reference-list type, but we do not error out. Instead, we return a NULL
value, which we catch in the GtkBuildable interface implementation to
get the actual object, and construct a reference list value.

There's still some ickyness around the value type that can only be
solved by having an attribute and role taxonomy.