Allow attachments on CiviMail

The following patch allow to add an attachment on a mailing.

The code might contain not that useful stuff, but the wizard is a bit too voodoo for me, I might have left things out of misunderstanding....

Limitations:

1) Once added, no way of deleting the attachement, beside copying the mailing.
2) When copying an event, doesn't copy the attachment.

Useful for my own usage IMMV

$svn diff CRM/Mailing > /tmp/php.diff
$svn diff templates/CRM/Mailing > /tmp/template.diff

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