core: Track system exit lock status

Used to determine if yuzu should confirm before pausing or stopping a game.

src/core/core.h

@@ -322,6 +322,10 @@ public:
 
     const Service::APM::Controller& GetAPMController() const;
 
+    void SetExitLock(bool locked);
+
+    bool GetExitLock() const;
+
 private:
     System();