Add a new command `speedbar-window'.

Add a new command `speedbar-window'.

Speedbar now can be opened in a window instead of a separate frame. The
frame remains the default.

* doc/emacs/frames.texi: Mention Speedbar window mode.
* doc/misc/speedbar.texi: Document 'speedbar-window'.
* lisp/speedbar.el
(speedbar-prefer-window): New user option. If t, the command `speedbar'
open the speedbar in a window.
(speedbar-window-dedicated-window): New user option. If t the window is
dedicated.
(speedbar-window-side):  New user option. The side of 'speedbar-window',
defaults to left.
(speedbar-window-default-width): New user option. The default size of
the 'speedbar-window'.
(speedbar-window-max-width): New user option. Limits the width of the
'speedbar-window'. The user can resize the window as desired, but this
option will be the width of the window when restored.
(speedbar--buffer-name): New variable, the buffer name used for both
'speedbar-frame-mode' and 'speedbar-window-mode'.
(speedbar--window): New variable, the window displaying 'speedbar-window'.
(speedbar--window-width): New variable, store the current width of
'speedbar-window'.
(speedbar-easymenu-definition-trailer): Now it is a function that
returns a different trailer for 'speedbar-frame' and 'speedbar-window'.
(speedbar): Now it is a function that calls 'speedbar-frame-mode', the
default or 'speedbar-window-mode' based on the value of
'speedbar-prefer-window'.
(speedbar-frame-mode): Before opening a frame, close 'speedbar-window'
if it is open.
(speedbar-frame-or-window): New function, returns 'frame', 'window'
or nil if speedbar is not open.
(speedbar-window): New alias for 'speedbar-window-mode'.
(speedbar-window-mode): Enable of disable 'speedbar-window'.
(speedbar-window--window-live-p): New function, return non-nil if the
'speedbar-window' is live.
(speedbar-window--buffer-live-p): New function, return non-nil if the
'speedbar-buffer' is live.
(speedbar-window--live-p): New function, return t if 'speedbar-window'
is open.
(speedbar-window-current-window): New function, return t if the selected
window is speedbar-window.
(speedbar-window--close): New function, close the 'speedbar-window'.
(speedbar-window--width): New function, return the current width of
'speedbar-window'.
(speedbar-width): New function, return the 'speedbar' of
'speedbar-frame-mode' of 'speedbar-frame-mode'.
(speedbar-set-mode-line-format): Use the new 'speedbar-width' function.
(speedbar-directory-buttons): Use the new 'speedbar-width' function.
(speedbar--speedbar-live-p): New function, returns t if
'speedbar-frame-mode' or 'speedbar-window-mode' are open.
(speedbar-timer-fn): Now handle 'speedbar-frame-mode' and
'speedbar-window-mode'.

(cherry picked from commit c1aab8c49b763abc482a6dbc402782bbe5370da0)