Merge branch '397_mouse_click_column_heading'

Merge branch '397_mouse_click_column_heading'

* 397_mouse_click_column_heading:
  Cleanup of code. Remove unused panel_field_t::use_in_gui variable
  Added indicators for current sort type and sort direction.
  Fixed segfault in 'Brief file list' listing mode
  Added reversing sort order ability if one keybind raised multiple times.
  Added keymaps for some sort types.
  Remove code for reverse search direction when toggle search types.
  Fixed behavior of  CK_PanelSelectSortOrder event.
  Remove unneded code
  Added handlers for panel sort keybind actions.
  Remove array of hardcoded format names from listmode.c
  Some changes for better toggle sort types of panel.
  Fixed length of panel header (current directory).
  Ticket #397: Mouse click on column heading sorts panel on that column