This gives an extra boost compared to commit
451c4ccc (see there for further details)
as it eliminates broadcasts to accessibility and thus obtaining text forwarders
and what not.

ScPostIt::CreateCaptionFromInitData() compared to that commit
from 145,055,889,118
to    63,386,691,509

Overall ScDocShell::SaveXML() from original state before that commit
Before: 370,433,720,090
 After: 107,468,180,524

Reduced to 29% or speed-up by factor 3.45

Change-Id: Ife86476c9654739ed9adf25e775f7568ea9934c2