Add ability to create and attach nodes with context menu

[jamesscottbrown]

IIUC, after adding a node representing an operation like Dispense, I need to add nodes representing parameters like the amount and dispenseVelocity. This requires scrolling down the list of node types on the left, selecting the appropriate one to add, and then dragging to add a connection.

Right-clicking on the ports on a node opens a context menu; it would be convenient if this included the option to create a new node of the appropriate type, connect it to the port, and position it appropriately in a dingle step.