Return the master placeholder this layout placeholder inherits from.

def _base_placeholder(self):
        """
        Return the master placeholder this layout placeholder inherits from.
        """
        base_ph_type = {
            PP_PLACEHOLDER.BODY:         PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.CHART:        PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.BITMAP:       PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.CENTER_TITLE: PP_PLACEHOLDER.TITLE,
            PP_PLACEHOLDER.ORG_CHART:    PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.DATE:         PP_PLACEHOLDER.DATE,
            PP_PLACEHOLDER.FOOTER:       PP_PLACEHOLDER.FOOTER,
            PP_PLACEHOLDER.MEDIA_CLIP:   PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.OBJECT:       PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.PICTURE:      PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.SLIDE_NUMBER: PP_PLACEHOLDER.SLIDE_NUMBER,
            PP_PLACEHOLDER.SUBTITLE:     PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.TABLE:        PP_PLACEHOLDER.BODY,
            PP_PLACEHOLDER.TITLE:        PP_PLACEHOLDER.TITLE,
        }[self._element.ph_type]
        slide_master = self.part.slide_master
        return slide_master.placeholders.get(base_ph_type, None)