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/msw/window.h

@@ -532,11 +532,9 @@ public:
     // check if mouse is in the window
     bool IsMouseInWindow() const;
 
-    // check if a native double-buffering applies for this window
+    virtual void SetDoubleBuffered(bool on) wxOVERRIDE;
     virtual bool IsDoubleBuffered() const wxOVERRIDE;
 
-    void SetDoubleBuffered(bool on);
-
     // synthesize a wxEVT_LEAVE_WINDOW event and set m_mouseInWindow to false
     void GenerateMouseLeave();