Port selection support

[bergie]

Ports should have a selected state which makes a ContextBar appear. The ContextBar should contain the following actions:

• Export / Stop exporting - makes the port available outside of the graph (and makes it unavailable inside the graph)
• Disconnect (for connected ports) - removes edges
• Set/edit IIP value (or open Node inspector?)

The ContextBar control should dismiss the selection.

See https://github.com/bergie/noflo/blob/master/CHANGES.md#030-december-19th-2012 about exported ports. We need an input for naming the exported port.

Alternatively these features can be implemented in the Node inspector as described in #62

[bergie]

The export stuff can be handled elsewhere, and the other interactions are already provided by the node and edge inspectors. Closing.