Shared interface for nodes

[gunar]

From GDocs:

Each node will respond to specific methods regardless of their internal representation? For example node.id, node.label, node.icon, node.toString(), node.handleSubmit

Also, it'd help with components being able to reference data fields used in other components. For example, for the new "Calculate" component, it'd be nice to be able to get type-completion for creating complex formulas. Just having an array of dataFields would be already useful for referencing, but we could also try and be smarter and differentiate between inputs and outputs.