Uses separate left/right handle images.

Other minor fixes:

 * Consolidates to one Handler for the webview.
 * Explicitly sets Locale to US when formatting floats for javascript
Adds dynamic updating of highlit region.


Improvements:
 * Dynamically sets highlit region while dragging. This makes the 
   user experience much better, if the browser can keep up.
 * Error messages sent from javascript include stack traces.

Still TODO
 * Too FLASHY. The highlight blinks while the user is adjusting it.
   Is this an inherent annoyance or is this fixable?
 * The popup is still all over the place.