[RCD] Sorting by message index

We need to implement sorting by message index. It should be simple on PHP side, but I don't know how to make sorting switch in UI. Currently we have sorting:

• by <message list column> ascending
• by <message list column> descending

and we need:

• by message index ascending
• by message index descending

So, 3-state switch in message list header (http://trac.roundcube.net/ticket/1485936) is not sufficient. Second option, use status column header for this is in my opinion not intuitive and we'll use probably this for different purposes in the future (e.g. column selection). I don't like also a third option, adding 'No.' column. Any ideas?