Bug 916774 - [Clock] Rename StopwatchPanel => Stopwatch.Panel, move Timer.Panel to timer_panel.js

- Stopwatch/Stopwatch.Panel
    - renamed StopwatchPanel to be Stopwatch.Panel, a Panel object as a property of Stopwatch
    - added comments to describe Stopwatch and Stopwatch.Panel
- Timer/Timer.Panel
    - moved Timer.Panel from timer.js to it's own timer_panel.js file
- Timer tests
    - moved Timer.Panel tests to its own timer_panel_test.js file
    - removed some unused vars in timer_test.js?