Add icon for sortable table columns that is currently in an unsorted state.

[henrychoy]

In a table with columns that are sortable and others that are not, it'll be good to add an icon to identify which columns are sortable without having to hover over each. The sortable/unsorted icon can be in the same position as the up/down arrow when sorted.

Can you please add this feature or let me know how I can manually add it myself? I can add my own icon based on the sortable property, but it isn't in the same position as the ascending/descending arrows.