sw: small code refactor

Created method DocxAttributeOutput::AddToAttrList which encapsulates
a piece of code that was widely repeated in that class:

  if( some attribute list does not exist )
      create the list

  add some value to the list
  add some other value to the list

Change-Id: Ibef031395927a7cedbf84a372008cb2e2677ae36
Reviewed-on: https://gerrit.libreoffice.org/7187
Reviewed-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Tested-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>