prs: refactor _rewrite_sldIdLst() to use add_sldId()
prs: refactor _rewrite_sldIdLst() to use add_sldId()

scanny/python-pptx,hoopes/python-pptx,biggihs/python-pptx,kevingu1003/python-pptx,cchanrhiza/python-pptx,AlexMooney/python-pptx