[RfC] Review writer mail merge fixes

Dear writer devs,

can you please review private/jmux/mailmerge-fixes-review, which I would
like to get merged. I can push those 25 patches to Gerrit, if this helps.

This branch contains all my mail merge bugfixes, without any speed
improvements. These improvements change some sematics, like SwPageDesc
and SwFrmFmts from vector => sorted_array, and will be in an additional
review, once finished.

As this fixes some very basic functions, most commits don't have a fdo
number assigned.

I used documents from the following bugs to check the implementation and
test mail merge correctness.


Bugfixes
========

fdo#34502 Field "Page Number" shows constant number instead of page count
fdo#62364 Mailmerge problem if list or numbering at the end of document
fdo#78611 Mail merge : corrupted paragraph numbering

fdo#81750 MAILMERGE: Input fields are duplicated and lost the first two
characters
which is in Gerrit (https://gerrit.libreoffice.org/#/c/10834/); seems I
can't have patches in a private branch and as a Gerrit review.


New features
============

fdo#70346 Section condition doesn't hide section with database fields
fdo#81782 MAILMERGE: Some fields (Title, Subject, Author) only printed
to the first document


Workaround
==========

fdo#80395 MAILMERGE: Lines move when using mail merge
fdo#80926 Moving paragraph-bound anchor on ODT file load

The layout code is still broken (see fdo#80926). The implemented
workaround just prevents some "fix anchor"-code to actually break the
document.


Thanks for your review and comments

Jan-Marek