Also get rid of the GetKeyOffset() and GetKeyValueLength() calls when
copying dictionaries: the reference already knows its offset and length,
so no need to call them. This makes the dictionary and the array
handling more similar.

Change-Id: I65936acfaf857636a8d83da3a4cec69289eb89d8
Reviewed-on: https://gerrit.libreoffice.org/36282Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>