so the user update links dialog can control their generation

SdrEmbedObjectLink becomes exposed to calc so it can
detect if the link dialog needs to be used to update
ole links.

Change-Id: Id1dd7ea17342140eab9307d546528747e3a98090
Reviewed-on: https://gerrit.libreoffice.org/28879Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>