The problem was that XMLTextFieldExport::ExportFieldAutoStyle() assumed
that the text of a field anchor is always the toplevel XText, which is
true in case of body vs header text, but false in case header text vs
text-in-table-in-header.

So add an UNO property which exposes the parent of a table cell, this
way text in header (regardless of it's in a table or not) will have the
same XText, leading to writing the necessary <text:user-field-decl>
element for the matching <text:user-field-get> definition.

Change-Id: I077b8d7e9dfae4062539894318637e266b925382
Reviewed-on: https://gerrit.libreoffice.org/73176
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>