In the tiled rendering case, the slides, no matter
if it is inserted or deleted, the part names always return
sequential names i.e. Slide 1, Slide 2, ..., Slide N.

However the client side needs to know what slides had been
deleted or inserted, so it is necessary to send the hash codes.

Change-Id: I0e9caeec660c3e42dd9f751bdce7690f9ad365a1
Reviewed-on: https://gerrit.libreoffice.org/24267Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>