Add API to hide the Slider value popup

With the latest Quick Controls material style 3.0 changes, Slider shows the value on top of the handle when pressed. There should be API to disable this, which in here is controlled with LabelFormatter https://github.com/material-components/material-components-android/blob/master/docs/components/Slider.md

Looking at SliderHandle.qml I didn't see how to set the label rectangle visible to false. I would set this in QQEM as it already shows the value on left side of the slider. Also the popup gets clipped for the first element of the list (as the list sets "clip: true" and popup isn't any external item). See the attached capture.