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

docs/changes.txt

@@ -94,6 +94,7 @@ All:
 
 All (GUI):
 
+- Add wxDataViewToggleRenderer::ShowAsRadio().
 - Improve stock items consistency and aesthetics (dhowland).
 - Fix bug with missing items in overflowing AUI toolbar (Maarten Bent).
 - Revert to left-aligning wxSpinCtrl contents by default.