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
Loic Blot