LOKit function "signDocument" can sign document without the need
to open them. This is useful to sign PDF documents after they are
created from a currently opened (ODF, DOCX) document.

This adds DocumentDigner to sfx2, which could also be used in
desktop LibreOffice without opening them and in further tests.

Change-Id: Id6f242e817f594aa672f94f9c6f9b34e4035d46a
Reviewed-on: https://gerrit.libreoffice.org/65767
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>