Core, VideoCommon: Fix crash at shutdown due to destructor ordering

Previously, PerformanceTracker registered a callback to be updated on
emulation state changes. PerformanceTrackers live in a global variable
(g_perf_metrics) within libvideocommon. The callback was stored in a
global variable in libcore. This created a race condition at shutdown
between these libraries, when the PerfTracker's destructor tried to
unregister the callback.
Notify the PerfTracker directly from libcore, without callbacks, since
Core.cpp already references g_perf_metrics explicitly. Also rename
Core::CallOnStateChangedCallbacks to NotifyStateChanged to better
reflect what it's doing.

Source/Core/VideoCommon/PerformanceMetrics.h

@@ -7,6 +7,7 @@
 #include <deque>
 
 #include "Common/CommonTypes.h"
+#include "Core/Core.h"
 #include "VideoCommon/PerformanceTracker.h"
 
 namespace Core
@@ -29,6 +30,7 @@ public:
 
   void CountFrame();
   void CountVBlank();
+  void OnEmulationStateChanged(Core::State state);
 
   // Call from CPU thread.
   void CountThrottleSleep(DT sleep);