Fix destruction of wxNativeContainerWindow in wxMSW.

WM_CLOSE was not processed at all for native windows wrapped by
wxNativeContainerWindow because we don't handle it ourselves at wxWindow level
but still mark it as processed in order to prevent DefWindowProc() from
destroying the window. Unfortunately this also prevented the original handler
for this message in the native window from being called.

Calling just the original handler and not the wxWidgets one is not ideal
neither but is much better as it allows to e.g. close MFC frames wrapped in
wxNativeContainerWindow whereas before this didn't work at all as WM_CLOSE was
completely ignored.

Also call the original handler for WM_DESTROY to avoid similar potential
problems with this message, even if it doesn't seem to create any with MFC.

Extend the mfc sample to show how a wxPanel can be embedded into the existing
CFrameWnd.

samples/mfc/mfctest.cpp

@@ -70,6 +70,8 @@
 #endif
 
 #include "wx/evtloop.h"
+#include "wx/nativewin.h"
+#include "wx/spinctrl.h"
 
 #include "resource.h"
 
@@ -121,6 +123,38 @@ public:
     wxDECLARE_EVENT_TABLE();
 };
 
+class MyPanel : public wxPanel
+{
+public:
+    MyPanel(wxWindow *parent, const wxPoint& pos)
+        : wxPanel(parent, wxID_ANY, pos)
+    {
+        wxSizer* const sizer = new wxFlexGridSizer(2, wxSize(5, 5));
+        sizer->Add(new wxStaticText(this, wxID_ANY, "Enter your &name:"),
+                   wxSizerFlags().Center().Right());
+        m_textName = new wxTextCtrl(this, wxID_ANY);
+        m_textName->SetHint("First Last");
+        sizer->Add(m_textName, wxSizerFlags().Expand().CenterVertical());
+
+        sizer->Add(new wxStaticText(this, wxID_ANY, "And your &age:"),
+                   wxSizerFlags().Center().Right());
+        m_spinAge = new wxSpinCtrl(this, wxID_ANY);
+        sizer->Add(m_spinAge, wxSizerFlags().Expand().CenterVertical());
+
+        wxStaticBoxSizer* const
+            box = new wxStaticBoxSizer(wxVERTICAL, this, "wxWidgets box");
+        box->Add(sizer, wxSizerFlags(1).Expand());
+        SetSizer(box);
+
+        // We won't be resized automatically, so set our size ourselves.
+        SetSize(GetBestSize());
+    }
+
+private:
+    wxTextCtrl* m_textName;
+    wxSpinCtrl* m_spinAge;
+};
+
 // ID for the menu quit command
 #define HELLO_QUIT 1
 #define HELLO_NEW  2
@@ -147,6 +181,18 @@ CMainWindow::CMainWindow()
     LoadAccelTable( wxT("MainAccelTable") );
     Create( NULL, wxT("Hello Foundation Application"),
         WS_OVERLAPPEDWINDOW, rectDefault, NULL, wxT("MainMenu") );
+
+    // Create a container representing the MFC window in wxWidgets window
+    // hierarchy.
+    m_containerWX = new wxNativeContainerWindow(m_hWnd);
+
+    // Now we can create children of this container as usual.
+    new MyPanel(m_containerWX, wxPoint(5, 5));
+
+    // An ugly but necessary workaround to prevent the container TLW from
+    // resizing the panel to fit its entire client area as it would do if it
+    // were its only child.
+    new wxWindow(m_containerWX, wxID_ANY, wxPoint(4, 4), wxSize(1, 1));
 }
 
 void CMainWindow::OnPaint()