Single-instance components

[bergie]

There are situations where a certain component can only be used once in a graph. Examples:

• MsgFlo components representing physical hardware
• NoFlo remote subgraphs

In these cases, the component is already "running", so adding it to the graph only makes its ports available in the graph. With such components there may be some required parameters (like node ID corresponding to MsgFlo role), and they can be added only once.

[bergie]

Decided to drop. Graph editor should still be able to design things, even if they can't be instantiated right away