[misc] Stepping backwards through panels


I recently started using Calcurse and it's awesome!

However, there is one minor UI annoyance that I find quite frustrating - pressing Tab will walk forward through the panels (calendar, day, todo), but pressing Shift-Tab won't walk through the panels in reverse. Shift-tab seems to be a pretty universal shortcut in all the applications I've encountered so far.

I've started messing around with the codebase and think I'm most of the way there in terms of implementing this, but can't figure out the final 10%. I wanted to see whether this would be a wanted feature before spending more time on it or asking for specific help.