ETree/ETable: Allow easier adding of columns for sorting

This extends behaviour of left-clicking column header which is not
used for sorting yet in an ETree/ETable in a way:
a) alone left-click behaves like before, all current sorting is dropped
   and the ETree/ETable is sorted only by that column
b) left-click with pressed Ctrl key adds the column as the first to sort by
c) left-click with pressed Ctrl+Shift adds the column as the last to sort by

1 files changed, 5 insertions, 0 deletions

diff --git a/e-util/e-table-sort-info.h b/e-util/e-table-sort-info.h
index 1b1191108a..47ab851c4a 100644
--- a/e-util/e-table-sort-info.h
+++ b/e-util/e-table-sort-info.h
@@ -119,6 +119,11 @@ void		e_table_sort_info_sorting_set_nth
 					 guint n,
 					 ETableColumnSpecification *spec,
 					 GtkSortType sort_type);
+void		e_table_sort_info_sorting_insert
+					(ETableSortInfo *sort_info,
+					 guint n,
+					 ETableColumnSpecification *spec,
+					 GtkSortType sort_type);
 void		e_table_sort_info_load_from_node
 					(ETableSortInfo *sort_info,
 					 xmlNode *node,