docx: Added a workaround until text:display="none" support.

According to MSWord 2007:
- Ignoring text:span elements with text:display="none" in the TextStyle.
- Ignoring text:a elements containing only hidden text:span elements.
- Ignoring empty text:p elements with text:display="none" in text-properties
of the paragraph mark.

TODO: If the content of a list-item is hidden, then content of the first
NOT hidden paragraph replaces it. From the implementation point of view
that paragraph inherits list related properties.