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/components/marginsinput.h

@@ -42,10 +42,6 @@ namespace BlackGui
             //! Current values of margins
             QMargins getMargins() const;
 
-        protected:
-            //! \copydoc QFrame::paintEvent
-            virtual void paintEvent(QPaintEvent *event) override;
-
         signals:
             //! Margins changed
             void changedMargins(const QMargins &margins);