void EZ_DoScroll( EZ_Widget *scrollbar, int offset, int absolute)
A scrollbar uses three state variables to scroll an object: total_size, page_size
and start_position (e.g. number of lines, number of lines in one page and the
index of the first line currently displayed).
EZ_DoScroll scrolls the object which scrollbar controls by the specified
offset. If absoulte is True, offset is treated as the absolute
position, otherwise, it is interpreted as the offset from the start_position.