Read a complex stroke (interactively) and return the stroke.
Optional PROMPT in minibuffer displays before and during stroke reading.
Note that a complex stroke allows the user to pen-up and pen-down. This
is implemented by allowing the user to paint with button 1 or button 2 and
then complete the stroke with button 3.
Optional EVENT is acceptable as the starting event of the stroke.
(fn &optional PROMPT EVENT)