Previously a default source had to be Audio/Source and a default sink
had to be Audio/Sink. This did not account for:
- monitor sources (Audio/Sink)
- duplex nodes (Audio/Duplex)
- virtual sources (Audio/Source/Virtual)
With this change the restriction is now on the number of input/output
ports, so any Audio/* node with output ports is a valid target to select
as a default source, while any Audio/* node with input ports is a valid
target for a default sink.
Fixes#60
