Show serial port selection menu for FirmataStream instances.

[damellis]

This may need to be done directly in ofApp, since FirmataStream can't inherit from BaseSerialInputStream.

[damellis]

Alternatively, we could potentially factor out a base class that does nothing but provide the list of serial ports and allow for the selection of a port. (That base class couldn't actually manage the serial port itself, since FirmataStream uses ofArduino while our other serial streams use ofStream.)