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.

src/msw/nativewin.cpp

@@ -32,6 +32,57 @@
 // implementation
 // ============================================================================
 
+// ----------------------------------------------------------------------------
+// wxNativeWindow
+// ----------------------------------------------------------------------------
+
+bool
+wxNativeWindow::Create(wxWindow* parent,
+                       wxWindowID winid,
+                       wxNativeWindowHandle hwnd)
+{
+    wxCHECK_MSG( hwnd, false, wxS("Invalid null HWND") );
+    wxCHECK_MSG( parent, false, wxS("Must have a valid parent") );
+    wxASSERT_MSG( ::GetParent(hwnd) == GetHwndOf(parent),
+                  wxS("The native window has incorrect parent") );
+
+    const wxRect r = wxRectFromRECT(wxGetWindowRect(hwnd));
+
+    // Skip wxWindow::Create() which would try to create a new HWND, we don't
+    // want this as we already have one.
+    if ( !CreateBase(parent, winid,
+                     r.GetPosition(), r.GetSize(),
+                     0, wxDefaultValidator, wxS("nativewindow")) )
+        return false;
+
+    parent->AddChild(this);
+
+    SubclassWin(hwnd);
+
+    if ( winid == wxID_ANY )
+    {
+        // We allocated a new ID to the control, use it at Windows level as
+        // well because we assume that our and MSW IDs are the same in many
+        // places and it seems prudent to avoid breaking this assumption.
+        SetId(GetId());
+    }
+    else // We used a fixed ID.
+    {
+        // For the same reason as above, check that it's the same as the one
+        // used by the native HWND.
+        wxASSERT_MSG( ::GetWindowLong(hwnd, GWL_ID) == winid,
+                      wxS("Mismatch between wx and native IDs") );
+    }
+
+    InheritAttributes();
+
+    return true;
+}
+
+// ----------------------------------------------------------------------------
+// wxNativeContainerWindow
+// ----------------------------------------------------------------------------
+
 bool wxNativeContainerWindow::Create(wxNativeContainerWindowHandle hwnd)
 {
     if ( !::IsWindow(hwnd) )