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