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.

docs/doxygen/mainpages/const_cpp.h

@@ -207,6 +207,7 @@ Currently the following symbols exist:
     decide whether some function should be overloaded for both
     <code>long</code> and <code>long long</code> types.}
 @itemdef{wxHAS_MULTIPLE_FILEDLG_FILTERS, Defined if wxFileDialog supports multiple ('|'-separated) filters.}
+@itemdef{wxHAS_NATIVE_WINDOW, Defined if wxNativeWindow class is available.}
 @itemdef{wxHAS_IMAGES_IN_RESOURCES, Defined if <a href="http://en.wikipedia.org/wiki/Resource_(Windows)">
     Windows resource files</a> or OS/2 resource files are available on the current platform.}
 @itemdef{wxHAS_POWER_EVENTS, Defined if wxPowerEvent are ever generated on the current platform.}