Regression from commit e2b260fc (sw: let
layout not mark embedded object as modified, 2014-06-03), an infinite
loop was caused by:

1) SwDoc::SetOLEObjModified() triggering the maOLEModifiedIdle Idle
2) which at the end called SwWrtShell::CalcAndSetScale()
3) which at the end called chart::ChartModel::setModified()
4) where chart code called back into SwDoc::SetOLEObjModified() via the
modification listener, and this happened again and again.

The original fix wanted to avoid marking the document as modified
without a user interaction, so fix the bug by only calling setModified()
if it prevents a not-modified -> modified transition. This keeps the
original bug fixed, but prevents the infinite loop, that is always a
modified -> modified transition.

Change-Id: I3b56a91afaacd3e0b7cb646a492fd15f1b5168ee