node: move Display trait impl on NodeData
Another small cleanup to move toward rctree (which implements Display trait on node).
I also pushed https://gitlab.gnome.org/pborelli/librsvg/tree/rctree if you are curious about where I am going with this and in case you have an opinion on the NodeExt trait approach I use instead of having a RsvgNode newtype.
I filed https://github.com/RazrFalcon/rctree/pull/6 for the fixes I need upstream before that branch can work