Ticket #????: refactoring of dir_list class and related code.

Initial commit: use GCompareFunc instead of sortfn.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Move sort_field member out from panel_sort_info_t structure.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Rename panel_sort_info_t to dir_sort_options_t

...and use it in routines of load and sort directory.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
NOT FINISHED: add dir_list::len member to keep number of items in list.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>