Add new wxTreeListCtrl class.

This is a facade for wxDataViewCtrl allowing to easily work with multi-column
trees, possibly with an optional checkbox in the first column. Its API is very
similar to wxTreeListCtrl and it provides a simple migration path from the
latter.

Add the class itself, documentation for it and minimal unit tests.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68916 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/itemid.h

@@ -36,6 +36,9 @@ public:
     Type GetID() const { return m_pItem; }
     operator const Type() const { return m_pItem; }
 
+    // This is used for implementation purposes only.
+    Type operator->() const { return m_pItem; }
+
     void Unset() { m_pItem = NULL; }
 
     // This field is public *only* for compatibility with the old wxTreeItemId