This patch introduces a setting 'Export placeholders' to File/Export as PDF.
    It works like the related printer setting.

Change-Id: I3a11a4601b1244b57e55c9b73b58116d355e105c
Reviewed-on: https://gerrit.libreoffice.org/24976Reviewed-by: Oliver Specht <oliver.specht@cib.de>
Tested-by: Oliver Specht <oliver.specht@cib.de>