slide.new put the body into the first non-title placeholder in shape order,
so a layout with a small body placeholder ahead of its content placeholder
(as Keynote exports "Title & Bullets") got the body in the small box and
left the content placeholder empty.
The body now goes to the main content placeholder: idx 1 first, then a
full-size placeholder over a half/quarter one, then a text placeholder over
a picture/chart/table one, keeping shape order on ties.
Fixes#725