rendernodeparser: Add support for reusing nodes

rendernodeparser: Add support for reusing nodes

We extend the syntax for nodes from:
  <node-type> { ... }
to
  <node-type> { ... }
  <node-type> <string> { ... }
  <string>;
where the first is the same as before, the 2nd defines a named node and
the last references a previously defined node.
Or to give an example:
  color "node" {
    bounds: 0 0 10 10;
    color: red;
  }
  transform {
    bounds: 20 0 10 10;
    child: "node";
  }
This will draw the red box twice, once at (0,0) and once at
(20,0).

The intended use for this is both shortening generated node files as
well as allowing to write tests that reuse nodes, in particular when
dealing with caches.