Added text-bubble slider. This is an extension of QSlider which
displays the slider value in the handle.
Added text-bubble slider support to the input widget.
Added text-bubble support to the output widget.
Fixed bug in slider widget, which did not invert the slider
when the actuator min and max values were inverted.
Found simpler way to create custom widget. All it required was
a promotion of the subclassed QSlider.