sync bookmarkmanager only if change was by process

This is an attempt to fix a bug that seems quite difficult to reproduce,
It seems there is a weird race condition linked to the sync of the two files
.local/share/user-places.xbel
.local/share/kfileplaces/bookmarks.xml
causing the bug
https://bugs.kde.org/show_bug.cgi?id=343735
all processes that are listening for changes in the bookmark manager will
try to sync their bookmarks with the shared ones (user-places.xbel)
and depending what process arrives first, some bookmarks may be lost

The same commit will happen on kdelibs4 as well

CCBUG:343735
REVIEW:122459