Factor out wxDataViewRendererBase::NotifyEditingStarted()

Reuse the same code from the generic and native GTK and OS X implementations
of wxDataViewCtrl instead of triplicating it.

This fixes a small discrepancy between the wxOSX version, which didn't see the
model pointer correctly in the generated event, and all the others, but mainly
paves way for future improvements.

include/wx/dvrenderers.h

@@ -180,6 +180,9 @@ public:
     // wxDVR_DEFAULT_ALIGNMENT.
     int GetEffectiveAlignment() const;
 
+    // Send wxEVT_DATAVIEW_ITEM_EDITING_STARTED event.
+    void NotifyEditingStarted(const wxDataViewItem& item);
+
 protected:
     // These methods are called from PrepareForItem() and should do whatever is
     // needed for the current platform to ensure that the item is rendered