LibGUI: Allow construction of vertical GToolBars

There's currently a small paint glitch for vertical toolbars due to the
way StylePainter::paint_surface() draws a MidGray line at the bottom of
whatever a "surface" is supposed to be.

Libraries/LibGUI/GToolBar.h

@@ -18,6 +18,7 @@ public:
 
 protected:
     explicit GToolBar(GWidget* parent);
+    explicit GToolBar(Orientation, GWidget* parent);
 
     virtual void paint_event(GPaintEvent&) override;