Add stopping (and maybe pausing) tasks

[aentwist]

Is your feature request related to a problem? Please describe.
As a user, I want to be able to stop infinitely running tasks without closing and reopening. Maybe pausing is also possible, so I can make minor adjustments and then pick up where I left off.

Describe the solution you'd like
Stop (and maybe pause) buttons/states

Additional context
Pausing is tricky - the app is stateful and always expects to be in a certain place. How can pausing be implemented without requiring the resumed state to be the exact same as the stopped state? Maybe pausing is a separate issue.