Ticket #3157: unify panel sort arrow's direction.

Fix inconsistency of the arrow's direction in the panel header line
across skins.
Change default value of "sort-sign-down" from "," (comma) to "." (dot).

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '3157_panel_sort_sign'

* 3157_panel_sort_sign:
  Ticket #3157: unify panel sort arrow's direction.