Updating i/o progress while saving Gnumeric XML file.

* src/xml-io.c
s/N_ELEMENTS_BETWEEN_UPDATES/N_INPUT_ELEMENTS_BETWEEN_UPDATES
(xml_write_cell_and_position): Update i/o progress every
N_OUTPUT_ELEMENTS_BETWEEN_UPDATES cells.
(xml_write_styles): Ditto for styles.
(gnumeric_xml_read_workbook): Better error reporting.
(workbook_get_n_elements): Internal function, calculating the number of
"elements" (styles and cells) in workbook.
(gnumeric_xml_write_workbook): Split saving to three stages: building
XML tree, dumping it to memory buffer, saving (compressed) buffer
contents to file. Update i/o progress regularly.
The second stage still freezes UI though :(