Renamed CGuiHandler to GameEngine

- class CGuiHandler is now called GameEngine to better describe its
functionality
- renamed global GH to more clear ENGINE
- GH/ENGINE is now unique_ptr to make construction / deconstruction
order more clear and to allow interface / implementation split
- CGuiHandler.cpp/h is now called GameEngine.cpp/h and located in root
directory of client dir

client/media/CSoundHandler.cpp

@@ -10,7 +10,7 @@
 #include "StdInc.h"
 #include "CSoundHandler.h"
 
-#include "../gui/CGuiHandler.h"
+#include "../GameEngine.h"
 #include "../CGameInfo.h"
 
 #include "../lib/filesystem/Filesystem.h"
@@ -319,7 +319,7 @@ void CSoundHandler::soundFinishedCallback(int channel)
 
 	if(!callback.empty())
 	{
-		GH.dispatchMainThread(
+		ENGINE->dispatchMainThread(
 			[callback]()
 			{
 				for(const auto & entry : callback)