LibGUI+SoundPlayer: Use 'decrease_slider_by_page_steps()' method

This method allow us to avoid repeating the pattern
'set_value(value() - page_step() * page_number)'.

Userland/Applications/SoundPlayer/SoundPlayerWidgetAdvancedView.cpp

@@ -152,7 +152,7 @@ void SoundPlayerWidgetAdvancedView::keydown_event(GUI::KeyEvent& event)
         m_volume_slider->increase_slider_by_page_steps(1);
 
     if (event.key() == Key_Down)
-        m_volume_slider->set_value(m_volume_slider->value() - m_volume_slider->page_step());
+        m_volume_slider->decrease_slider_by_page_steps(1);
 
     GUI::Widget::keydown_event(event);
 }

Userland/Libraries/LibGUI/Slider.cpp

@@ -106,7 +106,7 @@ void Slider::mousedown_event(MouseEvent& event)
             if (mouse_offset > knob_last_edge)
                 increase_slider_by_page_steps(1);
             else if (mouse_offset < knob_first_edge)
-                set_value(value() - page_step());
+                decrease_slider_by_page_steps(1);
         }
     }
     return Widget::mousedown_event(event);