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.
Shouldn't we add value to the list? `["value", "min", "max", "step"].forEach(this.notifyCurrentValue, this);`