Create (or destroy) horizontal and vertical scrollbars. The arguments
specify which you want (default: both). The scrollbars always have
minimum 0 and maximum 32767.
You must supply this method. It should return a tuple (x,
y) giving the current position of the scrollbars (between
0 and 32767). You can return None for either to
indicate the whole document is visible in that direction.
Supplied by you and called after user interaction. which will
be 'x' or 'y', what will be '-',
'-', 'set', '++' or '+'. For
'set', value will contain the new scrollbar position.
Auxiliary method to help you calculate values to return from
getscrollbarvalues(). You pass document minimum and maximum value
and topmost (leftmost) and bottommost (rightmost) visible values and
it returns the correct number or None.
Handles scrollbar interaction. If you override it call this method
first, a nonzero return value indicates the hit was in the scrollbars
and has been handled.