Add wxNativeWindow allowing to easily embed native widgets in wx.

Implement the class for wxMSW, wxGTK and wxOSX/Cocoa, show it in the widgets
sample and add documentation for it.

samples/widgets/native_wrapper.mm

@@ -0,0 +1,15 @@
+/////////////////////////////////////////////////////////////////////////////
+// Program:     wxWidgets Widgets Sample
+// Name:        native_wrapper.mm
+// Purpose:     Helper to compile native.cpp as Objective C++ code
+// Author:      Vadim Zeitlin
+// Created:     2015-08-02
+// Copyright:   (c) 2015 Vadim Zeitlin
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+// As explained in widgets.bkl, we need to compile native.cpp using Objective
+// C++ compiler, but there is no simple way to do it at the build system level,
+// so we just include that file from this file, which has .mm extension
+// ensuring that the correct compiler is used.
+#include "native.cpp"