Highlight compatible ports when dragging

[bergie]

When dragging a new connection from an input or output port, highlight the compatible ports visually (or show incompatible ports as "greyed out").

This means taking into account:

• Port data types noflo/noflo/#51 and noflo/noflo#84
• Port connection limits of regular port vs. ArrayPort, see noflo/dataflow-noflo#2
• Ports already having an IIP, see #62
• Ports that have been exported, see #64

This should be a fainter highlight than the one shown when dragging over a port in #37

[bergie]

Incompatible ports are de-emphasized