Currently all a11y notifications get send out immediately, which often
ends up formatting the document before it is ready. With the current
EnterBlockNotifications()/LeaveBlockNotifications() system it is difficult
to find all places that need blocking and any change in the a11y code
might require additional blocking in unpredictable places. By queueing
all notifications by default and only sending them out when the document
is ready, we can make sure that it can't be corrupted.

Change-Id: I9599c7b57eb5b8f8f0575de57fcc8bab171f78ff
Reviewed-on: https://gerrit.libreoffice.org/58703
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>