Don't discard highlighted annotations when merging paragraphs

All text nodes that are children of grouping elements should be counted as ODF content.

Fixes #666.
Details about document text content is here: http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1415196_253892949. WebODF doesn't actually follow the spec precisely, and uses a whitelist rather than a blacklist (issue #58 proposes the opposite).