[th/examples-show-wifi-networks] examples: rework "python/gi/show-wifi-networks.py" example

• only printing the scan list is not gonna cut it. It's usually stale, and we need to request a new scan.

• don't hard-code the GEnum and GFlags values that we understand. We have libnm, which provides us some meta information about the data. Use it.

• Some code cleanup.