MS3KOS2.0: Countdown Timer Mode

A common mode request has been a countdown timer - for example, the user enters the mode, selects a time period, and the MS3K immediately starts counting down - 15m, 10m, 5m, 1m, 30s, 20s, 10s, &etc.

I started programming this mode - this issue tracks further progress on integrating this mode into MS3KOS2.0.