accounting: fix grouping dialog loosing section category on next page
In xhtml_style, just after clicking next or previous page in the listbox buttons, the values selected in 'Section Category' and 'Section Category Strict' were not used, because they were used from request and not from as normal script parameters.
This fixes only the xhtml_style version, ERP5JS has another problem that dialog fields values are reset when going to next page, so it would also need this problem to be fixed.