Modification of Slide Master in Master Slide

l10n
#1

Hi all,

For information, I've accepted a change in terminology for Impress
because it was confusing for our users: there is currently Slide Master
and Master Slide used in UI, this is a functionality that is not easy to
understand, having two different names adds to it, so it's not only
cosmetic.
Here is an abstract of the ESC minutes:
+ Slide Master → Master Slide
          + concerned about the size of the change – help?; also MSO
uses ‘slide master’ (Kendy)
          + from l10n perspective, this is fine (Sophie)
            + 54 occurrences of “slide master” in helpcontent2 (Eike)
          + https://gerrit.libreoffice.org/#/c/40485/

Cheers
Sophie