- Edit control uses undocumented window style 0x0200 to detect is it a part
  of combobox. If so it calls GetDlgItem(hwndCombo, 1000) to get ComboLBox
  window handle (see comments for combo.c below).
- EDIT_CheckCombo - modified for correct handling of keyboard messages.
- Processing of WM_GETDLGCODE and WM_CHAR for VK_RETURN and VK_ESCAPE
  depends on whether listbox is dropped down. This prevents closing of dialog
  if listbox is dropped down and allows combobox to process these keyboard
  messages properly.