Revert "Join the emu thread in Core::Stop. Get rid of Core::Shutdown which did that before."

This reverts commit ba664b3293.

Added documentation to Core::Shutdown() to prevent breaking changes.

Source/Core/Core/Core.h

@@ -39,6 +39,7 @@ enum EState
 
 bool Init();
 void Stop();
+void Shutdown();
 
 std::string StopMessage(bool, const std::string&);