Add wxDataViewValueAdjuster

Add wxDataViewRenderer:: SetValueAdjuster() and a
wxDataViewValueAdjuster class. This can be used to customize rendering
of values depending on whether they are highlighted (selection) or not,
without having to implement an entire new custom renderer.

include/wx/osx/dvrenderer.h

@@ -93,6 +93,8 @@ protected:
     void SetEnabled(bool WXUNUSED(enabled)) wxOVERRIDE { };
 #endif
 
+    virtual bool IsHighlighted() const wxOVERRIDE;
+
 private:
     // contains the alignment flags
     int m_alignment;