Add a pause button

**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.