fix reading of section margins (bnc#875715)

The reason the section ended up with incorrect margins was because
that is computed as 'page margin' - 'section offset', and the order
of setting these up was backwards: First InsertSection() created
the section using the page margin of the old page, and only afterwards
SetSwFmtPageDesc() set the page margin that should be used.

Change-Id: I18ddcb0b5928879a2297b6caca766eab979abd45