Basic node/plug/connection colour scheme

This implements coloured drawing in the node graph. Colours are specified via metadata entries, meaning they can be specified at a high level for node types and named plugs on nodes, and at a lower level for specific instances of nodes. Default colours are set by a startup file, and per-instance node colours can be set using the UIEditor. I've just set up a very basic default config, and I wouldn't be surprised if people want to tweak it, or have better ideas about how things should look.