Provide "next step" button when halt is active #100

On the main timer-list screen, when a timer is not running, the start button is visible. When a timer is running, the stop and pause buttons are visible.

The problem with this is that when a halt step is encountered for a running timer, there's not a single button that can be pressed from the main list in order to move to the next step.

To address this problem, when a timer is in halt mode, it would be good to show three buttons: stop, pause, and next-step.