Clean-up of split filter implementations needed

[Stephan Bergmann]

With 
<http://cgit.freedesktop.org/libreoffice/core/commit/?id=5658600001a41c536cc1497eb7419512cb1353d8> 
"No need for two Excel filter services" and 
<http://cgit.freedesktop.org/libreoffice/core/commit/?id=3505f0e46c2e0a743ab5de9adfbec3044a4573a6> 
"Revert 'fdo#46808, Convert oox::PowerPointExport service to new style'" 
I had noted two service implementations that were each split in two for 
no apparent good reason, and those hacks were starting to grow.

In both cases, it was not immediately obvious to me how to fold the 
import and the export implementation into a single C++ class, though. 
(In the first case, I was able to remove the additional export service 
completely by forwarding to it from the import service directly in C++. 
  In the second case, that was not trivially possible, as the export 
service is implemented in sd and the import service in oox, and the 
latter cannot include from the former.)

It would be great if anybody with deeper insight into 
oox::core::XmlFilterBase etc. could clean this up further.

Stephan