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.

interface/wx/nativewin.h

@@ -0,0 +1,79 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/nativewin.h
+// Purpose:     wxNativeWindow documentation.
+// Author:      Vadim Zeitlin
+// Created:     2015-07-31
+// Copyright:   (c) 2015 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+/**
+    @class wxNativeWindow
+
+    Allows to embed a native widget in an application using wxWidgets.
+
+    This class can be used as a bridge between wxWidgets and native GUI
+    toolkit, i.e. standard Windows controls under MSW, GTK+ widgets or Cocoa
+    views. Using it involves writing code specific to each platform, at the
+    very least for creating the native window, but possibly also to handle its
+    events, but this class takes care of all the generic parts.
+
+    @note Check whether @c wxHAS_NATIVE_WINDOW is defined before using this
+        class as it is not available under all platforms.
+
+    For example, to embed a native GTK+ "light switch" control in a wxWidgets
+    dialog you could do the following:
+    @code
+        #include <wx/nativewin.h>
+
+        wxNativeWindow* switch = new wxNativeWindow(parent, wxID_ANY, gtk_switch_new());
+    @endcode
+    and then use @c switch as usual, e.g. add it to a sizer to layout it
+    correctly. Of course, you will still have to use the native GTK+ functions
+    to handle its events and change or retrieve its state.
+
+    See the "native" page of the widgets sample for another example of using
+    it.
+
+    @since 3.1.0
+
+    @library{core}
+    @category{ctrl}
+ */
+class wxNativeWindow : public wxWindow
+{
+public:
+    /**
+        Default ctor, Create() must be called later to really create the window.
+     */
+    wxNativeWindow();
+
+    /**
+        Create a window from an existing native window handle.
+
+        Notice that this ctor doesn't take the usual @c pos and @c size
+        parameters, they're taken from the window handle itself.
+
+        Use GetHandle() to check if the creation was successful, it will return
+        0 if the handle was invalid.
+
+        See Create() for the detailed parameters documentation.
+     */
+    wxNativeWindow(wxWindow* parent, wxWindowID winid, wxNativeWindowHandle handle)
+
+    /**
+        Really create the window after using the default ctor to create the C++
+        object.
+
+        @param parent A non-NULL parent window. For the platforms where the
+            parent is used for creating the native window (e.g. MSW), this must
+            be the wxWindow corresponding to the parent handle used when
+            creating the native window.
+        @param winid ID for the new window which will be used for the events
+            generated by it and can also be used to FindWindowById().
+        @param handle A valid native window handle, i.e. HWND under MSW.
+        @return @true if the creation was successful or @false if it failed,
+            typically because the supplied parameters are invalid.
+     */
+    bool Create(wxWindow* parent, wxWindowID winid, wxNativeWindowHandle handle);
+};