Optimisations

* Concurrently populate the list of .emlx/messages when searching a mailbox folder
* Use NSURL and resource keys when searching mailboxes for messages (to avoid hitting the disk multiple times)
* When checking if a mailbox contains children - assume there are messages after a single file has been found (don't load all the files to check there is more than 0)

... so yeah, I just replied to that gGroups post, and played with the plugin myself. Wow it (WAS!) slow ;-)

I still have not fixed #1921

Maybe if I do I'll actually start using the plugin :P