OpenXML: Exclude hidden powerpoint slides by default

Strangely, we exclude hidden word and excel content by default (with a switch for visibility), but not powerpoint.

Testcase attatched. (Slide 2 is hidden.)

We should exclude hidden slides by default, and make an option to display them. (Note that hidden text content within a visible slide is already supported, I think, by the existing "Translate Hidden Content" option.)