[WTL] 系统菜单中加入新项

1.在OnInitDialog中加入：  
// IDM_ABOUTBOX must be in the system command range.
  ATLASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
  ATLASSERT(IDM_ABOUTBOX < 0xF000);

  HMENU hMenu = GetSystemMenu(FALSE);
  CMenu sysMenu(hMenu);
  if (sysMenu.m_hMenu != NULL)
  {
   CString strAboutMenu;
   strAboutMenu.LoadString(IDS_ABOUTBOX);
   if (!strAboutMenu.IsEmpty())
   {
    sysMenu.AppendMenu(MF_SEPARATOR);
    sysMenu.AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu);
   }
  }

2.加入消息响应：
(1)  MSG_WM_SYSCOMMAND(OnSysCommand)
(2)
 void OnSysCommand( UINT nID, CPoint point)
 {
  if ((nID & 0xFFF0) == IDM_ABOUTBOX)
  {
   CAboutDlg dlgAbout;
   dlgAbout.DoModal();
  }
  else
  {
   //很重要，使用缺省处理
   SetMsgHandled(FALSE);
  }
 }