Fix ODF roundtrip of underlines.

2014-03-17  Andreas J. Guelzow <aguelzow@pyrshep.ca>

	* openoffice-write.c (odf_write_style_cell_properties): use
	extension to determine high or low underlines
	* openoffice-read.c (oo_style_prop_cell): read new gnm: attribute
	and make sure we reverse the mapping we use when we write