Creates a new shape for the system caret and claims ownership of the caret.
CreateCaret automatically destroys the previous caret shape, if any, regardless of which window owns the caret. Once created, the caret is initially hidden. To show the caret, the ShowCaret member function must be called.
The system caret is a shared resource. CWnd should create a caret only when it has the input focus or is active. It should destroy the caret before it loses the input focus or becomes inactive.