Make the attachment open API an official one

[jdede]

Move the access to the attachment opener from the experiment to the main API. Steps suggested by John Bieling:

1. create a bug in bugzilla to announce the new API and be able to track it
2. announce it on topicbox, so the broader add-on developer community can respond and work together on the API description (for collaboration it is useful to create a google doc and link that to the topicbox thread)
3. After having discussed the API on topicbox and you got feedback that it will be beneficial to include it Thunderbird, attach a patch to your bugzilla bug