[MERGE] Support Claws as a mail client.

[Gavin Panella]

This branch adds support for the Claws MUA in a similar way to how
KMail, Evolution, Mutt, Thunderbird, etc. are supported. In theory,
the XDGEmail client should be enough, but a bug in xdg-utils prevents
it from working correctly:

  Bug 291847 in xdg-utils "xdg-open mangles mailto: urls"
  https://bugs.edge.launchpad.net/ubuntu/+source/xdg-utils/+bug/291847

By also using the --attach option to claws-mail, it prevents the
display of a superfluous modal dialog in Claws that informs the user
that a file has been attached to the email.

As suggested by Aaron Bentley, I will also package this as a plugin so
that people using packaged versions of bzr (in Ubuntu, for example)
can use this sooner.

Thanks!
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