SessionSettings: Application Autostart: Made columns sortable.
requested to merge DarkTrick/xfce4-session:SessionSettings_ApplicationAutostart_SortableColumns into master
Details: (1) Added: Sort interface to XfaeModel (GTK_TYPE_TREE_SORTABLE) (2) Added: Sort function for each column (3) Renamed: xface_model_sort_items() -> xface_item_sort_default() (4) Fixed GTK bug to not realize column width correctly