This patch contains several minor changes to the native Excel export plugin (xls.php).
- make column header fixed on print (repeatRows) and when scrolling (freezePanes)
- split data on several worksheets when table contains > 65536 rows (Excel's limit)
- change "Put fields names in the first row" from text field to checkbox (like it is on other export plugins)
- $col renamed to $rowIndex (it counts rows, not columns)