For e-mail that arrives with no [class] identifier,
default to something useful.

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Not only should there be default values for the class,
but also for other properties. These defaults should
depend on the mail address, so that e.g. "issue-tracker"
could stand for the traditional email interface, and
"<productname>-issues" for issues with nosy set to
developers of this product.

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mail issues with a already used subject are rejected ?

if author is the 
same
 . append to already exiting one
else
 . make a new issue

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The mailgw already does check titles when the nodeid isn't
supplied. With the class now being defaulted, the title will
be used to look up followups when no info is supplied.

Implemented the basic behaviour - the more complex stuff
(setting default properties) can wait for a better config
system.

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*ahem* sorry, misread gruber's requirement. I think the
behaviour should be that if the title matches an existing issue:
 1. if there's a Re/Fwd/AW prefix, append
 2. otherwise, make a new issue (requires removing the key
    restriction on the title property)

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This is now done, for the two default schemas.