Merged to 3.0.5.

A compromise.  We do this check only if we are viewing all (i.e. 
subscribed and unsubscribed) folders.  There is no way to check for 
the existence of (possible) children if doing a subscribed only view 
without explicitly doing a all folders check under that node, which is 
exactly what we are trying to avoid in the first place - namely, 
accessing any levels of the IMAP Tree we do not expressly need to 
depending on the user's current view.

When initializing the imap tree and building the tree elements, in 
isOpen() we should not only check for in the nav_polled preference if 
a folder is expanded, but also if the folder has children.

I have an obviously broken nav_polled array that markes a folder as 
open even though it doesn't have any children. This might happen for 
example if deleting subfolders of expanded folders with a different 
mail client.