Extract reusable part of the MFC sample in a header

Allow reusing this functionality from outside the library, it can he
useful if an MFC window needs to be embedded into a wx application (or
vice versa).

Also use a better wxEntryStart() overload as a side effect, this should,
in particular, fix the problem with command line arguments processing in
mixed MFC/wx applications pointed out in a comment in the sample
previously.

include/wx/msw/mfc.h

@@ -0,0 +1,137 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/msw/mfc.h
+// Purpose:     Helpers for applications using both wxWidgets and MFC
+// Author:      Julian Smart, Vadim Zeitlin
+// Created:     2017-12-01 (mostly extracted from samples/mfc)
+// Copyright:   (c) 2017 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_MSW_MFC_H_
+#define _WX_MSW_MFC_H_
+
+#ifndef __AFXWIN_H__
+    #error "MFC headers must be included before including this file."
+#endif
+
+#include "wx/app.h"
+#include "wx/evtloop.h"
+#include "wx/window.h"
+#include "wx/msw/winundef.h"
+
+// ----------------------------------------------------------------------------
+// MFC window class wrapping a window created by wxWidgets
+// ----------------------------------------------------------------------------
+
+class wxMFCWnd : public CWnd
+{
+public:
+    explicit wxMFCWnd(wxWindow* w)
+    {
+        Attach(w->GetHWND());
+    }
+
+    ~wxMFCWnd()
+    {
+        // Prevent MFC from destroying the wxWindow.
+        Detach();
+    }
+};
+
+// ----------------------------------------------------------------------------
+// MFC application class forwarding everything to wxApp
+// ----------------------------------------------------------------------------
+
+class wxMFCWinApp : public CWinApp
+{
+public:
+    BOOL InitInstance() wxOVERRIDE
+    {
+        if ( !CWinApp::InitInstance() )
+            return FALSE;
+
+        if ( !wxEntryStart(m_hInstance) )
+            return FALSE;
+
+        if ( !wxTheApp || !wxTheApp->CallOnInit() )
+            return FALSE;
+
+        if ( !InitMainWnd() )
+            return FALSE;
+
+        return TRUE;
+    }
+
+    int ExitInstance() wxOVERRIDE
+    {
+        delete m_pMainWnd;
+
+        if ( wxTheApp )
+            wxTheApp->OnExit();
+
+        wxEntryCleanup();
+
+        return CWinApp::ExitInstance();
+    }
+
+    // Override this to provide wxWidgets message loop compatibility
+    BOOL PreTranslateMessage(MSG *msg) wxOVERRIDE
+    {
+        wxEventLoop * const
+            evtLoop = static_cast<wxEventLoop *>(wxEventLoop::GetActive());
+        if ( evtLoop && evtLoop->PreProcessMessage(msg) )
+            return TRUE;
+
+        return CWinApp::PreTranslateMessage(msg);
+    }
+
+    BOOL OnIdle(LONG lCount) wxOVERRIDE
+    {
+        BOOL moreIdle = CWinApp::OnIdle(lCount);
+
+        if ( wxTheApp && wxTheApp->ProcessIdle() )
+            moreIdle = TRUE;
+
+        return moreIdle;
+    }
+
+protected:
+    // This virtual method can be overridden to create the main window using
+    // MFC code. The default implementation relies on wxApp::OnInit() creating
+    // a top level window which is then wrapped in an MFC window and used as
+    // the main window.
+    virtual BOOL InitMainWnd()
+    {
+        wxWindow* const w = wxTheApp->GetTopWindow();
+        if ( !w )
+            return FALSE;
+
+        // We need to initialize the main window to let the program continue
+        // running.
+        m_pMainWnd = new wxMFCWnd(w);
+
+        // And we need to let wxWidgets know that it should exit the
+        // application when this window is closed, as OnRun(), which does this
+        // by default, won't be called when using MFC main message loop.
+        wxTheApp->SetExitOnFrameDelete(true);
+
+        return TRUE;
+    }
+};
+
+// ----------------------------------------------------------------------------
+// wxWidgets application class to be used in MFC applications
+// ----------------------------------------------------------------------------
+
+class wxAppWithMFC : public wxApp
+{
+public:
+    void ExitMainLoop() wxOVERRIDE
+    {
+        // There is no wxEventLoop to exit, tell MFC to stop pumping messages
+        // instead.
+        ::PostQuitMessage(0);
+    }
+};
+
+#endif // _WX_MSW_MFC_H_