Qt: Improve handling of exclusive fullscreen loss

Exclusive fullscreen will be re-requested automatically when the window
regains focus.

src/duckstation-qt/qthostinterface.h

@@ -180,6 +180,7 @@ public Q_SLOTS:
 private Q_SLOTS:
   void doStopThread();
   void onHostDisplayWindowResized(int width, int height);
+  void onHostDisplayWindowFocused();
   void doBackgroundControllerPoll();
   void doSaveSettings();
 
@@ -276,4 +277,5 @@ private:
   bool m_is_rendering_to_main = false;
   bool m_is_fullscreen = false;
   bool m_is_exclusive_fullscreen = false;
+  bool m_lost_exclusive_fullscreen = false;
 };