Import [Random:ContinentCulture] civ choice for custom start

[flabbert]

"Random Culture: African" etc; choose civ based on native culture from game menu. Examine how Ashes of Erebus manages this to select Random Alignment, mechanism should be importable. CvGame.cpp in AoE, starting from "int iRndCiv = GC.getDefineINT("RANDOM_CIVILIZATION");"