Slider: fix Observable related issues, and Slider.html sample file

1. Use notifyCurrentValue() so that Slider computes correct value on startup, when the user doesn't value, and also doesn't specify min, max, or step.

2. Don't call this.notifyCurrentValue("value") after setting this.value; it's meaningless.

3. Change sample file to use observe() on slider value, so that it reports programmatic modifications in addition to changes initiated by the user.   (Other sample files should also be changed maybe.)

I'm not sure about that refresh() method though, maybe it needs more work.

See ibm-js/decor#8.
Try it on IE10, you'll see the Set button doesn't work without it.   At least that's what I saw.

Also, see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/button or the HTML spec, which states that the default type is "submit".   You don't want this to be a submit button, right?