pagedesc: It's better when MakePageDesc returns a pointer.

This removes many uses of GetPageDesc(sal_uInt16).

Change-Id: I9660e30d7fccd4336422ba03b19087321a7ae973
pagedesc: Consolidate FindPageDescByName() and FindPageDesc().

Change-Id: I18d570509cd3f3b65a19a12a9df0b1d5db901d04
pagedesc: We have DelPageDesc() that takes OUString as param directly.

Change-Id: I616f89716a6e337c647cfb880a4e675cd1b65993