[ticket/12029] Extend width of ACP sidebar 20 pixels

Extending it 20 pixels, to me looks right. Any further and it simply
looks too wide. 20px is enough to squeeze in a few more letters 
in the rare instances where needed, without compromising the
look of the ACP.

http://tracker.phpbb.com/browse/PHPBB3-12029

PHPBB3-12029