GSlider: Add support for vertical sliders.

You now have to pass an Orientation to the GSlider constructor. It's not
possible to change the orientation after construction.

Added some vertical GSliders to the WidgetGallery demo for testing. :^)

DevTools/VisualBuilder/VBWidgetRegistry.cpp

@@ -107,7 +107,7 @@ static GWidget* build_gwidget(VBWidgetType type, GWidget* parent)
         return bar;
     }
     case VBWidgetType::GSlider: {
-        auto* slider = new GSlider(parent);
+        auto* slider = new GSlider(Orientation::Horizontal, parent);
         slider->set_range(0, 100);
         slider->set_value(50);
         return slider;