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.

samples/mfc/mfctest.h

@@ -9,11 +9,7 @@
 #ifndef __MFCTEST_H__
 #define __MFCTEST_H__
 
-/////////////////////////////////////////////////////////////////////////////
-
-// CMainWindow:
-// See hello.cpp for the code to the member functions and the message map.
-//
+// CMainWindow: just a normal MFC window class.
 class CMainWindow : public CFrameWnd
 {
 public:
@@ -31,30 +27,30 @@ private:
     class wxNativeContainerWindow* m_containerWX;
 };
 
-// A dummy CWnd pointing to a wxWindow's HWND
-class CDummyWindow: public CWnd
+#if START_WITH_MFC_WINDOW
+
+// There is no need to define an application class if the default behaviour of
+// using the wxWindow created in wxApp::OnInit() as main window is acceptable,
+// but if we want to create the initial window in MFC, we need this class in
+// order to override InitMainWnd() in it.
+class SampleMFCWinApp : public wxMFCWinApp
 {
-  public:
-    CDummyWindow(HWND hWnd);
-    ~CDummyWindow(void);
+protected:
+    BOOL InitMainWnd() wxOVERRIDE
+    {
+        // Demonstrate creation of an initial MFC main window.
+        m_pMainWnd = new CMainWindow();
+        m_pMainWnd->ShowWindow( m_nCmdShow );
+        m_pMainWnd->UpdateWindow();
+
+        return TRUE;
+    }
 };
 
-/////////////////////////////////////////////////////////////////////////////
+#else // !START_WITH_MFC_WINDOW
 
-// CTheApp:
-//
-class CTheApp : public CWinApp
-{
-public:
-    BOOL InitInstance();
-    int ExitInstance();
+typedef wxMFCWinApp SampleMFCWinApp;
 
-    // Override this to provide wxWidgets message loop
-    // compatibility
-    BOOL PreTranslateMessage(MSG *msg);
-    BOOL OnIdle(LONG lCount);
-};
-
-/////////////////////////////////////////////////////////////////////////////
+#endif // START_WITH_MFC_WINDOW/!START_WITH_MFC_WINDOW
 
 #endif // __MFCTEST_H__