Multi-column sorting for Tables

[kkoreilly]

Discussed in https://github.com/cogentcore/core/discussions/1076

^{Originally posted by runrc July 29, 2024} To enhance the usability of Tables, it would be useful to allow multi-column sorting via selection of multiple column headers. Clicking on a column header would allow up, down or no-sorting. Subsequent columns could be added to the sorting criteria by clicking. Columns can be removed by clicking the column header to select no-sorting.