Improve chart roundtrip through ODF. [#728197]

2014-04-18  Andreas J. Guelzow <aguelzow@pyrshep.ca>

	* openoffice-read.c (odf_apply_style_props): set default
	(od_style_prop_chart): handle interpolation-skip-invalid
	* openoffice-write.c (odf_write_interpolation_attribute):
	write interpolation-skip-invalid
	(odf_write_gog_style_chart): write interpolations for GogLineSeries