[CEGUI] Model/View widgets

by Trimiton for WorldForge

The project involves unifying the underlying implementation of several listed widgets (Tree, ListBox, ComboBox, etc.) so they use the same base model. Also, the aim is to allow a modular model/view approach (separation of concerns) on getting the actual listed widgets data, without requiring manual and duplicate operations on the widgets and the underlying model data (e.g.: adding/removing items from a list).