LibGUI: Add a GItemView class.

This is a GAbstractView subclass that implements a icon-based view onto
a GModel. It still need a bunch of work, but it's in basic usable shape.

LibGUI/GModel.h

@@ -42,7 +42,7 @@ public:
         const Font* font { nullptr };
     };
 
-    enum class Role { Display, Sort, Custom, ForegroundColor, BackgroundColor };
+    enum class Role { Display, Sort, Custom, ForegroundColor, BackgroundColor, Icon };
 
     virtual ~GModel();