(speedbar-item-info-file-helper): Add optional arg

(speedbar-item-info-file-helper): Add optional arg
of the file whose info we want to display.
(speedbar-easymenu-definition-trailer) Fix list issue w/ customize.
(speedbar-add-mode-functions-list) Improve doc.
(speedbar-line-token) New function.
(speedbar-dired) Fix order of directories in -shown-directories.
(speedbar-line-path): Default return is default-directory
(speedbar-buffers-line-path): Return is dir name only.
(speedbar-mode-functions-list): New variable.
(speedbar-mouse-item-info): Rewrote to be a replaceable fn.
(speedbar-item-info-file-helper, speedbar-item-info-tag-helper
speedbar-files-item-info speedbar-buffers-item-info): New functions.
(speedbar-fetch-replacement-function,speedbar-add-mode-functions-list):
New functions.
(speedbar-line-file): Broke out part that fetches file from a line.
(speedbar-line-text): New function extracted from speedbar-line-file.
(speedbar-line-path): Converted into a replaceable function.
(speedbar-files-line-path, speedbar-buffers-line-path): New functions.