GameUI refactor (part 1/X): GameUI object creation + GameUIFlags move to GameUI

GameUI refactor (part 1/X): GameUI object creation + GameUIFlags move to GameUI

Game class is too huge and has too specialization on various subjects, like UI, formspecs, client, renderer. Start to move UI related things to GameUI object and cleanup them

Other improvements:
* updateChat: more performance on error messages by remove string copies
* Initialize all game class members in definition instead of constructor (with nullptr instead of NULL)
* Drop unused Client::show{GameChat,GameHud,Profiler,GameFog}
* Add GameUI unittests