rename SvRef::AddRef to AddFirstRef

to make it's intended purpose clearly distinguishable from AddNextRef

Change-Id: I5da780b48b19fd873667b648031bc394113f953b
Reviewed-on: https://gerrit.libreoffice.org/11763
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>