Support new function creation

[cphyc]

There should be an easy way to create a new node.This can be achieved

• using either a "new function" command which would create a new function with a default template.
• by allowing new nodes to be created on the graph. It could be initialized with an empty universal socket which type would become specialized once anything is connected into it. On connection, a new universal socket would be created, etc.
• by creating a default node that takes as input the previously selected node's output.