- Edit text must be selected only if CB is in focus.
- If CB has edit control we have to call CBUpdateEdit to update its
  contents. Invalidating textRect will not force updating of child
  edit control, obviously.
- We have to protect ourselves from changing selection in listbox when
  we receive listbox notification. So LBN_SELCHANGE -> CBUpdateEdit ->
  EN_CHANGE will not reselect item in the lisbox.