Don't let new messages steal focus

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When a new message arrives, a chat window is opened that also steals focus. 
This happens every time except when focus is already in another chat window.

It should never steal focus, since that may lead to information being sent to 
wrong participants. 

I would also be nice of there was a configuration setting that prevented it 
from popping up a new window when new messages arrives. That is, always let 
ijabbar have the same behaviour as it does when new messages arrives and it is 
allready in another chat window.

These two issues are important when integrating ijab with roundcube, as 
described in the link below:
http://rahul.amaram.name/blog/2010/09/05/integrating-ijab-roundcube

Original issue reported on code.google.com by [email protected] on 7 Nov 2010 at 5:34