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

interface/wx/dataview.h

@@ -2272,6 +2272,9 @@ public:
 
     This class is used by wxDataViewCtrl to render toggle controls.
 
+    Note that "toggles" can be represented either by check boxes (default) or
+    radio buttons.
+
     @see wxDataViewCheckIconTextRenderer
     @library{wxadv}
     @category{dvc}
@@ -2292,6 +2295,24 @@ public:
     wxDataViewToggleRenderer(const wxString& varianttype = GetDefaultType(),
                              wxDataViewCellMode mode = wxDATAVIEW_CELL_INERT,
                              int align = wxDVR_DEFAULT_ALIGNMENT);
+
+    /**
+        Switch to using radiobutton-like appearance instead of the default
+        checkbox-like one.
+
+        By default, this renderer uses checkboxes to represent the boolean
+        values, but using this method its appearance can be changed to use
+        radio buttons instead.
+
+        Notice that only the appearance is changed, the cells don't really
+        start behaving as radio buttons after a call to ShowAsRadio(), i.e. the
+        application code also needs to react to selecting one of the cells
+        shown by this renderer and clearing all the other ones in the same row
+        or column to actually implement radio button-like behaviour.
+
+        @since 3.1.2
+     */
+    void ShowAsRadio();
 };