Export centre-across-selection in a way LO can understand. [#726010]

2014-04-19  Andreas J. Guelzow <aguelzow@pyrshep.ca>

	* openoffice-read.c (oo_cell_start): watch for fake spans
	* openoffice-write.c (odf_cellspan_is_empty): new
	(odf_write_cell): simulate center-accross-selection as merges