Finish page: added XPUI_FINISHPAGE_CANCEL_ENABLE, which defaults to off. (Warning, default behavior change! - per MUI compat)

!ifndef XPUI_SYSDIR
  !define XPUI_SYSDIR "${NSISDIR}\Contrib\ExperienceUI"
!endif
!include "${XPUI_SYSDIR}\XPUI.nsh"