Powerpoint: Support raw openxml in pptx writer.

This allows raw openxml blocks and inlines to be used in the pptx
writer.

A few caveats:

1. It's up to the user to write well-formed openxml. The chances for
corruption, especially with such a brittle format as pptx, is pretty
high.

2. Because of the tricky way that blocks map onto shapes, if you are
using a raw block, it should be the only block on a slide (otherwise
other text might end up overlapping it).

3. The pptx ooxml namespace abbreviations are different from the docx ooxml
namespaces. Again, it's up to the user to get it right. Unzipped
document and ooxml specification should be consulted.

Closes: #4976

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