Vertical scrollbar widget.

rc is the resource class of the widget. ".(un)focused" sets the (un)focused style of the widget. ".barstyle" can be filled or outline. ".track" is a bool to display a central track line.

default_event_handler when true (the default) installs the mouse_event and scroll_key_event handlers.

width (resp. height) defines the prefered thickness of the scrollbar.