Don't allow starting a new timer when one is already running

[mmalecki]

No description provided.

[fnogatz]

Alright, six years passed 😧 In the mean time, I consider this a feature rather than a bug – I am frequently starting new timers while another one is already running, but this is intended. So I will close this issue here. (Maybe we should print a warning when starting a new timer when one is already running?)