FeatureRequest: RadioGUI / add RDF detection

[hbeni]

Would be cool to be able to enable RDF in RadioGUI:

• [x] add a new main menu entry to open the new RDF window
• [x] add a new window for RDF; which displays all registered radios
  • [x] add a checkbox to enable RDF for given radio
  • [x] display detected direction for incoming signals in a small circle

Needed Backend additions:

• [x] add UDP server thread
• [x] add parser for the RDF part

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Could be handy to a) test the RDF feature; and b) as a simple ATC client for small groups / small airports w/o radar

Ref: https://github.com/hbeni/fgcom-mumble/blob/master/client/plugin.spec.md#rdf-data

[hbeni]

Implemented in a68bc34