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A slider widget is a sunken rectangle on which a button moves in one dimension, either
vertically or horizontally.  A slider appears as a linear scale with optional numerical
labes and an optional short label.  Sliders are typically used to edit a numerical value.
The user adjust the value by clicking mouse button 1 in the slider or by dragging the slide
button with mouse button 1. If a slider has callbacks, they are
invoked whenever its numeric value changes
