Apparently LibreOffice behaves strangely when the first thing in a
document is a 0-height paragraph, and can move into the page header
and get stuck there.
For around this by making fake paragraphs "very small" instead of 0,
avoiding the issue.  "very small" is sufficiently small to have no
visual effect.

This has a few advantages

 * Easier to use from a Python program
 * No temporary files
 * ZIP content order is preserved

This avoids creating unnecessary duplicates that will likely affect
display.

Avoid dealing with the hierarchy of frames that we wouldn't really
process anyway, to avoid any possible behavioral changes.

Rely on XSLT keeping the last attribute in case of duplicates.

It was here for debugging, doesn't really help, and is kind of
irrelevant now we generate some styles ourselves.

Since we generate valid sections with a name, the placeholder
paragraphs don't seem to be needed anymore.

We cannot remove them entirely because their style will sometimes
affect more than just the paragraph, but we should make sure it
doesn't account for vertical space as it was likely to align with
the frames.