use_cases.text

User is within the activity:
- User can start a timer
- User can stop a timer
- The timer shall go off when the time has ran out
- User can leave the activity by pressing home (pause)
- User can leave the activity by pressing back (kill)

User has left the activity
- The timer still executes in the background
- User can re-enter the activity 

USer has reentered the activity
- GUI has to be updated to most recent time
- User is in the activity