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/DolphinQt/Settings.cpp

@@ -36,7 +36,7 @@
 Settings::Settings()
 {
   qRegisterMetaType<Core::State>();
-  Core::SetOnStateChangedCallback([this](Core::State new_state) {
+  Core::AddOnStateChangedCallback([this](Core::State new_state) {
     QueueOnObject(this, [this, new_state] { emit EmulationStateChanged(new_state); });
   });