Fix FPS counter and Game Window speed % breaking on pause/unpause

-Add pause state to FPSCounter.
-Add ability to have more than one "OnStateChanged" callback in core.
-Add GetActualEmulationSpeed() to Core. Returns 1 by default. It's used by my input PRs.

Source/Core/DolphinNoGUI/MainNoGUI.cpp

@@ -216,7 +216,7 @@ int main(int argc, char* argv[])
     return 1;
   }
 
-  Core::SetOnStateChangedCallback([](Core::State state) {
+  Core::AddOnStateChangedCallback([](Core::State state) {
     if (state == Core::State::Uninitialized)
       s_platform->Stop();
   });