Previously we assumed that the Page object's dictionary has no Annots
key. Now detect if that's not true, and in that case don't just copy of
the whole dictionary (as part of the incremental update), instead copy
it in two steps, so we can insert our reference in the middle.

This is needed (but not enough alone) to be able to sign a PDF document
multiple times.

Change-Id: Ia5bf993320428eef80551e7e9cc7bfb2b858db7f
Reviewed-on: https://gerrit.libreoffice.org/30257Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>