refs #419, reviewed style sheet paintEvent code

* removed for QWidget
* added for QDialog
http://stackoverflow.com/questions/37952348/enable-own-widget-for-stylesheet

src/blackgui/dropsite.h

@@ -65,9 +65,6 @@ namespace BlackGui
         //! \copydoc QWidget::dropEvent
         virtual void dropEvent(QDropEvent *event) override;
 
-        //! \copydoc QWidget::paintEvent
-        virtual void paintEvent(QPaintEvent *event) override;
-
     private slots:
         //! Style has been changed
         void ps_onStyleSheetsChanged();