[MlMt] 'Default' account

[Igor Karpov]

Hi!

Got two IMAP sources defined in MailMate, Google & Yahoo!. How to set my 
Google mail account as 'default' one? I mean this account should be used 
every time when I am starting the new letter?

I've checked the output of `defaults read com.freron.mailmate`, but 
nothing seems to me as a good candidate for such settings. Looks 
MailMate uses some heuristics to determine which account to use...

Regards,
Igor