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

Merged Thomas Haller requested to merge th/examples-show-wifi-networks into main
  • 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.

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