Enable or disable speedbar. Positive ARG means turn on, negative turn off.
A nil ARG means toggle. Once the speedbar frame is activated, a buffer in
`speedbar-mode' will be displayed. Currently, only one speedbar is
supported at a time.
`speedbar-before-popup-hook' is called before popping up the speedbar frame.
`speedbar-before-delete-hook' is called before the frame is deleted.
(fn &optional ARG)