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/gtk/dvrenderer.h

@@ -64,6 +64,8 @@ public:
     // different from the editor control widget for the custom renderers
     virtual GtkWidget* GtkGetEditorWidget() const;
 
+    void GtkSetCurrentItem(const wxDataViewItem& item) { m_itemBeingRendered = item; }
+
 private:
     // Change the mode at GTK level without touching m_mode, this is useful for
     // temporarily making the renderer insensitive but does mean that GetMode()
@@ -74,6 +76,8 @@ protected:
     virtual void SetAttr(const wxDataViewItemAttr& attr) wxOVERRIDE;
     virtual void SetEnabled(bool enabled) wxOVERRIDE;
 
+    virtual bool IsHighlighted() const wxOVERRIDE;
+
     virtual void GtkOnCellChanged(const wxVariant& value,
                                   const wxDataViewItem& item,
                                   unsigned col);
@@ -93,6 +97,9 @@ protected:
     // doing this
     bool m_usingDefaultAttrs;
 
+    // the item currently being rendered
+    wxDataViewItem m_itemBeingRendered;
+
 protected:
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxDataViewRenderer);
 };