function NewMDIClient( AParent: PControl; WindowMenu: THandle ): PControl;
Creates MDI client window, which is a special type of child window, containing all MDI child windows, created calling NewMDIChild function. On a form, MDI client behaves like a panel, so it can be placed and sized (or aligned) like any other controls. To minimize flick during resizing main form having another aligned controls, place MDI client window on a panel and align it caClient in the panel.
Note: MDI client must be a single on the form.