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.
The thing it was changed to in 4199f6c was incorrect. If I'm reading that commit correctly, the intent was to restrict this function to only checking the text length of things that are considered part of the document's main text body. In WebODF, ``isGroupingElement`` is the key place that contains a list of these known containers (though, that is missing a few I noticed a few days ago... but that is another, undiscovered bug).