DOCX import: fix missing underline in comment text

Regression from fb5ee5c9953635a423d3102b901e409d15800096 (sw: Add
support for different grab bags at PropertyMap., 2013-11-29),
PropertyMap::GetPropertyValues() assumed all XTextRange implementations
support the new property, which is not true for at least editeng.

Change-Id: Ib5657be522d30f203cecbbbae74d6594cef984fb