Some minor UI improvements

* buttons right aligned
* min-height in qss
* removed "public slots" where not needed
* QMessageBox when changing widget style (because it is so slow)

src/blackgui/components/settingsguicomponent.h

@@ -41,7 +41,6 @@ namespace BlackGui
             //! Hide opacity elements
             void hideOpacity(bool hide);
 
-        public slots:
             //! GUI Opacity 0-100%
             void setGuiOpacity(double value);