Activates a different MDI child window.
This member function sends the WM_MDIACTIVATE message to both the child window being activated and the child window being deactivated.
This is the same message that is sent if the user changes the focus to an MDI child window by using the mouse or keyboard.
An MDI child window is activated independently of the MDI frame window. When the frame becomes active, the child window that was last activated is sent a WM_NCACTIVATE message to draw an active window frame and caption bar, but it does not receive another WM_MDIACTIVATE message.