agents are now in Agents group, even if the server tells us it's in another group

I tried to make gajim more HCI friendly. ERRORS use Error dialog and not warning dialog. Need core implementation too. Also other fixes. plz see a comment for you yann (sre)

remove some ugly hack for autoconnection

use spawn for fork/execve

Code is cleaner & faster now. Hope I didn't introduce any bugs. Next step: combine *EVERYTHING* not just formatting ;P. I rewrote the algo for that in recursion, and u'll see it in the next commit I hope