Ciao,
this patch adds speedbar-window-mode as an alternative to the default 
speedbar-frame-mode. Calling speedbar-window-mode will close speedbar-frame-
mode if it is open, and vice versa.
speedbar-window-mode, thanks to display-buffer-in-side-window, can be placed 
on the left side, right side but also on the top or bottom. 

Thank you.

Vincenzo