Completely remove IGameCallback class

- CClient now inherits directly from CPrivilegedInfoCallback, like
IGameCallback did before. However CClient no longer needs dummy
implementation of IGameEventCallback
- CGObjectInstance hierarchy now uses CPrivilegedInfoCallback for
callback. Actual events can only be emitted in calls that receive
IGameEventCallback pointer, e.g. heroVisit
- CGameHandler now inherits directly from both CPrivilegedInfoCallback
and IGameEventCallback as it did before via IGameCallback

lib/mapObjects/IMarket.cpp

@@ -140,7 +140,7 @@ int IMarket::availableUnits(const EMarketMode mode, const int marketItemSerial)
 	}
 }
 
-IMarket::IMarket(IGameCallback *cb)
+IMarket::IMarket(CPrivilegedInfoCallback *cb)
 	:altarArtifactsStorage(std::make_unique<CArtifactSetAltar>(cb))
 {
 }