Document that wxDataViewCtrl::GetItemRect() now works in wxGTK

Remove the note about it being non-implemented from the docs and mention
this in the change log.

interface/wx/dataview.h

@@ -1461,8 +1461,6 @@ public:
             If non-@NULL, the rectangle returned corresponds to the
             intersection of the item with the specified column. If @NULL, the
             rectangle spans all the columns.
-
-        @note This method is currently not implemented in wxGTK.
     */
     virtual wxRect GetItemRect(const wxDataViewItem& item,
                                const wxDataViewColumn* col = NULL) const;