Use km/h for Russian planes, knots for others

[István Váradi]

Make the setting available via the Flight object, which in turn should read it from the aircraft. Later, if it becomes selectable, we could still read it from the flight, which would read it from the GUI.

The following entities should consider the unit:

• The V₁, V_R, V₂, V_Ref entries
• Any speed values in the logs
• Look for the string "knot" in i18n.py

Care should be taken, that all speed data logged should include the unit!