If you like layering things on top of each other, then this commit
message is for you. So it's possible to have a PPTX file in the
following wrappers:

- wrap PPTX in a binary OLE2 container
- wrap that in an OLE1 container
- wrap that in an RTF fragment
- wrap that in an XHTML fragment (in a ReqIF file)

Turns out that only the RTF and OLE1 unwrapping was missing, the rest
worked already, so implement the missing piece in a new SwReqIfReader
namespace.

Finally extend OleEmbeddedObject to be able to read its native data
stream when the object is opened, reading it from the storage would
fail, as the object already opened the storage stream.

Change-Id: I2934c9fb7474e981ff6bb2f7eb253a3a86cfd98b
Reviewed-on: https://gerrit.libreoffice.org/51772Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>