The ValueNode class was initially designed to store read-only property trees. It is useful to also provide mutable access. Add non-const adapters and iterators and adapters. This allows obtaining a mutable ValueNode instance when traversing a tree. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com> Reviewed-by: Isaac Scott <isaac.scott@ideasonboard.com>