Add aerotow support

[hbeni]

Adds standard aerotow capability

• Glider presses CTRL+O behind the plane to hook into the Cessna
• Cessna pilot can release hook anytime by pressing SHIFT-O (Moved Domelight keybind SHIFT-O to o, as SHIFT-O is the default aerotow release)

After merging:

• add c172 to the towing overview
• add aircraft/Glider tug to C172P infobox template type
• add a short section to the C172P wiki page that it can aerotow (example: Cub)

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Tested with ASK21:

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Fix #1361

[wlbragg]

Thank you @hbeni for sharing this with the c172p. I haven't had much time to work on FG. I'm hoping to get back to work in the not to distant future.