fix slider Observable related issues, and tooltip in sample

1. Use notifyCurrentValue() so that Slider computes correct value on startup, when the user doesn't specify min, max, or step.

2. Don't call this.notifyCurrentValue("value") after setting 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.