When removing paragraphs with mail mere fields, both field type reference
and node reference could get invalid, because field type will be destroyed
when its last field is gone; and node will be destroyed if it is in a fly
with anchor in another node which gets destroyed.

To avoid use-after-delete, we will use an SwClient on field types, thus
detecting if a collected field type got destroyed; iterating over fields
using SwIterator is safe, because removing a node with fields would update
the iterator.

Change-Id: Id8b555ef7015b13ab70ebb41845d34c477ac6b31
Reviewed-on: https://gerrit.libreoffice.org/59164
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>