Sets the focus rectangle to the item at the specified index in a multiple-selection list box.
BOOL bScroll = TRUE
Specifies the zero-based index of the item to receive the focus rectangle in the list box.
If this value is 0, the item is scrolled until it is fully visible. If this value is not 0, the item is scrolled until it is at least partially visible.
LB_ERR if an error occurs.
If the item is not visible, it is scrolled into view.
// Set the caret to be the middle item.
ASSERT(m_myListBox.GetCaretIndex() == m_myListBox.GetCount()/2);