closeAllWindows seems to take no effect - use exit instead

Looks like it does not work, because there was never a window or any other window on top of QApplication.
.exit() should let us out of the main loop.

cura/CuraApplication.py

@@ -405,7 +405,7 @@ class CuraApplication(QtApplication):
         if main_window is not None:
             main_window.close()
         else:
-            self.closeAllWindows()
+            self.exit(0)
 
     ## A reusable dialogbox
     #