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.