Reset the wxDataViewItem being edited once it's not edited any more

No real changes, just some cleanup to ensure that the item being edited stored
in wxDataViewRendererBase never refers to an item which is not, actually,
being edited any longer.

include/wx/dvrenderers.h

@@ -202,7 +202,7 @@ protected:
     wxString                m_variantType;
     wxDataViewColumn       *m_owner;
     wxWeakRef<wxWindow>     m_editorCtrl;
-    wxDataViewItem          m_item; // for m_editorCtrl
+    wxDataViewItem          m_item; // Item being currently edited, if valid.
 
     // internal utility, may be used anywhere the window associated with the
     // renderer is required