[BUG] : mailbox node load/save config

[Emmanuel <e allaud wanadoo fr>]

	Hi all,
I found that we do not load/save the config of mailbox nodes. I think it's 
been forgotten : save function saves a lot of thing and the load one only 
loads the name, and neither get called anyway.

Moreover the default setting for sort is NONE, I think we should put the 
default to a sort by date, it seems more natural (remark that in general 
you won't really notice that there is not sorting done because eg a mbox 
file is in date order, almost).
If someone who knows the code better than me could take care of this (I'm 
struggling with balsa-index for now).
Bye
Manu