Access by AnchoredObject of a deleted SwLayoutFrm

as demonstrated by abi10075-1.doc

just register the AnchoredObjects in the SwLayoutFrm and
inform them when the SwLayoutFrm goes away.

This crash was triggered by "1e113cb7604e1509e7d598a9be329f1f7b6e9322" import
different first page header/footer from doc. But that commit is blameless.

Change-Id: Ia079cc635a81dff1ccbf740641f441aa784328a4
(cherry picked from commit b376eacdfae11f5d39eb7011efe67390d9f495e7)
Reviewed-on: https://gerrit.libreoffice.org/6622
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>