Safely close database connections and delete backends

Also fix NewClosure leak caused by disconnected object signals

src/core/mainwindow.h

@@ -243,6 +243,7 @@ signals:
   void Raise();
 
   void Exit();
+  void DoExit();
 
   void HandleNotificationPreview(OSD::Behaviour type, QString line1, QString line2);
 
@@ -262,6 +263,8 @@ signals:
   void LoveButtonVisibilityChanged(bool value);
   void Love();
 
+  void ExitFinished();
+
  private:
 
   void SaveSettings();