Add wxWindowBase::SetDoubleBuffered() stub

This method was already provided by wxGTK and wxMSW, but not wxOSX nor
any other ports.

Provide a stub for it in wxWindowBase to allow user code to call it on
all platforms, there is no harm in that even if it doesn't (and can't)
do anything under macOS.

include/wx/window.h

@@ -1065,6 +1065,9 @@ public:
         // adjust DC for drawing on this window
     virtual void PrepareDC( wxDC & WXUNUSED(dc) ) { }
 
+        // enable or disable double buffering
+    virtual void SetDoubleBuffered(bool WXUNUSED(on)) { }
+
         // return true if the window contents is double buffered by the system
     virtual bool IsDoubleBuffered() const { return false; }