Display connected port differently on the graph view

[djdeath]

When looking at the graph, the user should be able to tell when a port can't be connected to any other port.
For example, a port on which the user has set an initial should be display in red, to let the user know that nothing else can be connected to it. That would prevent the user wondering whether he/she forgot to connect something.

[bergie]

Similarly we should de-emphasize incompatible ports (based on type). Dataflow had this feature meemoo/dataflow#63

cc/ @forresto

[bergie]

Related to #75

[bergie]

Ports with IIPs, ports with connections, and ports with no connections are now shown differently