Add wxDataViewCtrlBase::SetAlternateRowColour()

Previously this method was only available in the generic wxDataViewCtrl,
move it to the base class to make it possible calling it in portable
code and document it.

Closes #14617.

include/wx/generic/dataview.h

@@ -277,10 +277,11 @@ public:
 
     virtual bool SetHeaderAttr(const wxItemAttr& attr) wxOVERRIDE;
 
-    // These methods are specific to generic wxDataViewCtrl implementation and
+    virtual bool SetAlternateRowColour(const wxColour& colour) wxOVERRIDE;
+
+    // This method is specific to generic wxDataViewCtrl implementation and
     // should not be used in portable code.
     wxColour GetAlternateRowColour() const { return m_alternateRowColour; }
-    void SetAlternateRowColour(const wxColour& colour);
 
     // The returned pointer is null if the control has wxDV_NO_HEADER style.
     //