node: move code to NodeData
Just moving code around. I want to make RsvgNode smaller moving toward the goal to either make it a trait implemented by rctree::Node or a small newtype that derefs into Node or something like that...
Basically I have the feeling that:
- we are now relying on adding an impl for NodeRef which is a type defined in the tree module
- it might be interesting to split RsvgNode into multiple traits (eg Element trait that has id(), class() etc and a Trait used when building the tree and setting attributes