Despite the fixes for Bug 714317, fetching PREVIEW without also downloading the complete message is still rather broken. To fix it properly, we need to have the IMAP BODYSTRUCTURE on hand, so we know the content type and structure of the message's parts in advance, and hence can actually target the correct part.

In addition, having this would allow us in the future to optionally/incrementally download body parts for e.g. lazily fetching bodies and attachments (Bug 714094).

I'm not sure when is the best time to fetch it though. We could make it a requirement of ImapDB.Folder, in which case we'll be able to examine the structure from the get-go, or we could let the conversation list require it, so when it makes its second pass to update the preview, it will at least be able to properly request the preview.