[Evolution] Minor feature request: Default mailbox store format

[Ron Guerin <ron vnetworx net>]

I'd like to request that users should be able to select a default
mailbox store format.

I for example, use maildir exclusively.  But for every new folder I
create, I must convert it to maildir format manually.  I should be able
to select maildir as my default mail store format in the preferences.

Thanks!