We need an API, or to extend an existing one, that lets us open messages in a tab, just like you can in the UI.

At the moment there's very little in the way of tab opening apart from content tabs, so we might want to consider calendar / chat / mail folders etc as part of this.

Note: Conversations uses this for its "Open in classic reader" functionality.

Though it may be useful to have for other extensions that want to be able to provide UI which opens a specific message/folder.