Add wxDataViewToggleRenderer::ShowAsRadio()

This allows showing radio buttons in wxDataViewCtrl easily and natively.

Notice that this approach, adding an extra function to the existing
renderer class instead of creating some new wxDataViewRadioRenderer (see
https://github.com/wxWidgets/wxWidgets/pull/809), was finally chosen
because it is simpler to implement and, more importantly, because it
will be more natural to generalize if/when we also add a 3-state
check/radio renderer.

Closes https://github.com/wxWidgets/wxWidgets/pull/853

include/wx/gtk/dvrenderers.h

@@ -107,6 +107,8 @@ public:
                               wxDataViewCellMode mode = wxDATAVIEW_CELL_INERT,
                               int align = wxDVR_DEFAULT_ALIGNMENT );
 
+    void ShowAsRadio();
+
     bool SetValue( const wxVariant &value ) wxOVERRIDE;
     bool GetValue( wxVariant &value ) const wxOVERRIDE;