LibGUI: Don't draw left and right side of surfaces that span entire window.

In other words, if a surface stretches from the left side of the window
all the way to the right side, skip shading and highlighting the sides.
This makes widgets blend together just slightly with the window. :^)

LibGUI/GWidget.h

@@ -139,6 +139,8 @@ public:
     bool is_visible() const { return m_visible; }
     void set_visible(bool);
 
+    bool spans_entire_window_horizontally() const;
+
 private:
     virtual bool is_widget() const final { return true; }