  This note is an extended version of my presentation at the "Focused Research
Workshop on Exterior Di{\S}erential Systems and Lie Theory" held at the Fields
Institute in Toronto, December 9-13, 2013. Some ideas and constructions of this
note crystallized during my communication with Anthony D. Blaom. I am deeply
grateful to him and also to P.J. Olver for his constant help and encouragement.
