LibGUI+VisualBuilder: Support custom editing widgets for property values.

Implemented this by letting GAbstractViews provide a GModelEditingDelegate
for a given index, which then knows how to create and setup a custom widget
appropriate for the data type being edited.

DevTools/VisualBuilder/VBWidgetPropertyModel.h

@@ -10,6 +10,7 @@ public:
     enum Column {
         Name = 0,
         Value,
+        Type,
         __Count
     };