RTF import: fix dobxpage before dptxbx

The problem was that dobxpage arrived first, set HoriOrientRelation to
FRAME, then dptxbx tried to apply defaults, which overwrote the already
set HoriOrientRelation. Fix this by only applying properties which are
not set yet.

Change-Id: I108f3363a2758eee0242533fe92e511e8c522b68