Allow pausing the timer

[doersino]

As of now, pressing t toggles the timer shown in the presenter notes window. That is,

• if the timer is inactive, it is started and shown, and
• if the timer is active, it is hidden and reset to 0.

I can imagine situations where it would be beneficial to pause the timer. The following interaction would make sense to me: If the user presses t,

• if the timer is inactive, it is started and shown (nothing new here), and
• if the timer is active, it is paused (this could be visually further emphasized by making it blink).
• To reset the timer, the user would have to press tt (i.e. twice in quick succession, perhaps within half a second), which would hide and reset the timer.

Since pausing the timer is implementation-wise not completely trivial, I might not address this unless someone wants me to (leave a comment!).