Base class for HScrollbar and VScrollbar

The Scrollbar widget is an abstract base class for HScrollbar and VScrollbar. It is not very useful in itself.

The position of the thumb in a scrollbar is controlled by the scroll adjustments. See Adjustment for the fields in an adjustment - for Scrollbar, the "value" field represents the position of the scrollbar, which must be between the "lower" field and "upper - page_size." The "page_size" field represents the size of the visible scrollable area. The "step_increment" and "page_increment" fields are used when the user asks to step down (using the small stepper arrows) or page down (using for example the PageDown key).

 |  GObject
 |   +----Object
 |         +----Widget
 |               +----Range
 |                     +----Scrollbar
 |                           +----HScrollbar
 |                           +----VScrollbar