Bug 909394 - [Clock] Implement Stopwatch view

- StopwatchPanel
    - Subclasses Panel
    - Handles View 'visibilitychanged' event for show/hide behavior
    - Creates an internal Stopwatch object
    - Starts, pauses, resumes and resets the stopwatch when respective buttons pressed
    - Displays the lap time returned by the stopwatch when respective button pressed
    - Toggles Start/Pause/Resume buttons on user click
    - Toggles Lap/Reset buttons on user click
    - Lap times display in list down below with index, capped at 99
- Stopwatch
    - Made accessor for isStarted
    - Fixed bug with lapTimes not correctly taking pauses into account
- Stopwatch Unit Test
    - Added tests for isStarted()
    - Added test for lap() after a pause
- Issues Raised / Possible new bugs
    - Should 'Lap' button be disabled after 99 laps are reached?
    - Should time readout grow to handle hours?