Add a pause button

[HadrienMP]

Is your feature request related to a problem? Please describe.
Some pairs like to use longer times to switch and when they are interrupted they feel robbed of their time at the keyboard.

Describe the solution you'd like
Add a pause button, it should probably pause the pomodoro as well

Describe alternatives you've considered
Stop and restart. Reject interruptions. But this is a feature that many people seem to want.