docx: paragraph/text styles inheritance Kung Fu

* Let's set line-height=100% in the default paragraph style if the corresponding part
of the w:spacing element is not provided.

* Do not interpret a default named style (w:default attribute in the w:style element)
as additional information for the defaults. The "Normal" style for example on top
of the defaults was applied to any paragraph which was not correct. Then finally
the corresponding style referred by a paragraph was applied. So the "Normal" style
messed up the formatting.

* Any paragraph and text chunk that does not provide the style information should used the
corresponding default named style (w:default attribute in the w:style element).

* paragraph/text styles for footnotes/endnotes belong into content.xml, not styles.xml
and also make use of the default named styles.

* headers/footers also make use of default named styles.