Refactor treadmill controls

[HoLuLuLu]

• Use two buttons
  • left button for start/pause/resume
    • start button will switch to pause button if workout started by clicking on it
    • pause button will switch to resume button if workout paused by clicking on it
    • resume button will switch to pause button if workout resumed by clicking on it
  • right button for stop
    • button is deactivated until workout started
    • clicking on it will stop the workout and reset the button layouts
• Use current workout state from WorkoutStateManager
• Add timer to display countdown in app (will interrupt if pad starts earlier)
• Add comments on some checks

[HoLuLuLu]

This should affect #54

[hughesjs]

Hi mate, I have been so busy this last few weeks, I've only just seen your PRs come in, sorry! I'll try and get them all reviewed this weekend. Thanks for your contributions!