Return the nominal column and row in POSITION, measured in characters.
The column and row values are approximations calculated from the x
and y coordinates in POSITION and the frame's default character width
For a scroll-bar event, the result column is 0, and the row
corresponds to the vertical position of the click in the scroll bar.
POSITION should be a list of the form returned by the `event-start'
and `event-end' functions.