Slider Element
Extends TextElement.
This element cannot be built and is the basis of other sliders.
The handle position is between 0.0 and 1.0, from left to right.
Class methods:
getHandlePosition: Gets the handle position.setHandlePosition: Sets the handle position.getSmoothHandlePosition: Gets the smoothened handle position.isHandleFocused: Returnstrueif the handle is focused.