[MERGE] Mail clients listed in a registry

[Neil Martinsen-Burrell]

Here is a patch to make mail clients extensible by plugins by listing  
them in a registry. While Bazaar may reasonably try to support all  
popular mail clients, there will always be niche programs and  
platforms that we can't support.  There will also be times where some  
vaporware exists to support a certain client, but it hasn't landed  
yet.  (This is currently the case with Mail.app.)

The patch makes it possible to add a mail client from a plugin as the  
attached plugin demonstrates.  (This email sent using Mail.app and  
that plugin.)  Test coverage is still adequate as we currently test  
the getting of mail_clients from Config objects for every built-in  
client.

-Neil

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