It is expected that if you load an OLE2 storage (from reqif-xhtml) and
you save it as ODT, the OLE2 data is not modified. This was almost the
case, but we insisted on removing the OLE2 preview from the storage.

Add a new flag to OleEmbeddedObject, so import filters can opt in for
not modifying the OLE2 data.

[ The nice situation is that we already had a test file which had a
preview stream in the OLE2 storage, so we can easily assert there that
the size doesn't change. ]

Change-Id: I9b8b29f015dd4f2513e51a1066767218580cb5d8
Reviewed-on: https://gerrit.libreoffice.org/63381Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins