* boot.c (excel_save): Set up io_progress before calling ms_excel_check_write
and ms_excel_write_workbook.

* ms-excel-read.c
(ms_excel_read_sheet): Added IOContext argument,
call value_io_progress_update on every loop iteration.
(ms_excel_read_workbook): Set up io progress message and helper.

* ms-excel-write.c
(pre_cell): Update io progress.
(pre_blanks): Update io progress at every row.
(gather_styles): Set up io progress helper.
(write_block): Update io progress after every write_cell call.
(ms_excel_write_workbook): Set up io progress helper.
(ms_excel_check_write): Copy IOContext ptr into ExcelWorkbook struct.

* ms-excel-write.h (struct _ExcelWorkbook): Added io_context.