Follow HTML5 progress element specification

This PR updates the ProgressBar widget to follow the HTML5 specification [1] of the progress element wrt  value, max and position attributes behavior:
* changes value and max behavior,
* removes the min property,
* introduce the position property.

Also contains these changes:
* Enforces template attach points.
* Removes call to validateProperties() in refreshRendering: now enforces init value validation in startup().
* Switch to JSDoc.

[1] http://www.w3.org/TR/2012/WD-html5-20121025/the-progress-element.html