(speedbar-scan-subdirs): New option.

(speedbar-scan-subdirs): New option.
(speedbar-file-lists): Don't ignore file-name case on Unix and use dolist.
(speedbar-insert-files-at-point): Take an extra argument and use it to optionally find out if a subdir is empty.  Also unreadable files don't get expand buttons.
(speedbar-directory-plus): New image (bitmap already existed unused).
(speedbar-expand-image-button-alist): Use it.