use bookmarks to mark mailmerge parts in a mailmerge document (fdo#80823)

Instead of page numbers, which
- was somewhat fragile (and broken, as it was actually off-by-one)
- required repeated re-layout of the increasingly large document, making
  mailmerge awfully slow. The re-layout is not removed by this commit, as
  it needs further checking whether it can be removed.
See the bugreport for details.

Change-Id: Ib09bd5f5a6a549c3d38ca40b0f32c0d2831fdd4c
compute page offset for mailmerge based on source document rather than target

This removes another need for doing repeated and expensive layouts of the target
document.

Change-Id: Id78bc3ccc71c17e42f858dc9660866b9c94dea3a
do not repeatedly relayout mailmerge document after each change

When mailmerge uses a single document for all the generated documents, doing
a layout of this increasingly large document after every generated document
addition was getting slower and slower (no wonder). My recent changes should
have removed all needs for any layout in progress, so it should be enough
to just once calculate the final layout. The final layout still appears to be
needed, leaving it as it is to get layout done on the fly puts e.g. 2-page
documents on wrong starting page (maybe just invalidating something would fix
that).
Debug build shows warnings from vdraw.cxx about moving anchor from an invalid
page because of broken layouting, but those were there already before, so I
assume those are not really significant (or at least made worse by this commit).

Change-Id: I62601ad6dccaa008783d1ce34c9e4f66f9621a56
make it possible to check mailmerge output files in unittests

Change-Id: Ifa54445c8f46bb2ed5539870c5fce1a2e62beef7
do not register MM unittest databases with a prefix

Since that makes SwDBManager::IsDataSourceOpen() return false because
of different data sources (than the one the document was tested with manually),
which prevents DocumentFieldsManager::UpdateExpFlds() from updating fields
properly.

Change-Id: I66f5f79d9fd39204eabb247aa92ec11acef56903
basic test of mail merge working

Change-Id: Ia72d1b2827bfb38ab39d7d0df04908874d25e124
make parseDump() properly handle xml node content

Change-Id: Ibb8859908c8e73377e1ee20b041b9ae76955d076
properly discard dumped layout on test document (re)loads

Change-Id: I8673810e8c04cd128a691ee1c1e8c4731108b605
unit test to verify basic mail merge layout for 2 pages

Change-Id: I2172d0185a2481700ffe152e58ebd5bd9c6a4fab
name of mailmerge marks is actually irrelevant

UNO bookmarks have internals names and ignore this

Change-Id: I37baa4c9bcf9cec37f91e3b1d0fb2fad322ceda8
unittest verifying that mail merge correctly sets up marks for sub-documents

I.e. the UNO bookmarks that are used for marking where each generated document
starts inside the one large single document.

Change-Id: I8069a3110cacd34b25aff6739ab248f490c3fa6a