[sylpheed:36066] Fix suggestion on folder retrieval

[Javier]

Hi,

I discovered quite late (I think it was availble from October) that
Microsoft offered IMAP for Hotmail and Outlook free accounts and I
decided to add them to Sylpheed.

And I found that Sylpheed did what didn't with other accounts: tried
to create a Trash folder because it didn't find one and couldn't
"attach" to a Trash folder for the Trash folder. In this service the
Trash is Deleted.

As this IMAP service has Trash as a reserved folder name, Sylpheed
sent the command to create its own Trash folder, that became a child
of Inbox.

(I'm sorry if I don't attach a log file, I didn't make a backup of it :S)

Afterall there was no harm. Just set the properties of Deleted to
Trash and delete Trash Inbox child. But this can become a problem if,
by mistake, you don't delete that folder correctly.

My suggestion for Sylpheed is to avoid the creation of folders if
they didn't match to the expected to avoid any problems. In fact, if
I remember correctly, this kind of problem existed with Yahoo
accounts and Sent box, but, in this case, Sylpheed didn't create a
new Sent folder.

In my opinion, Sylpheed must only retrieve folders and let user decide
the type of folder for each one if Sylpheed is unable to match
expected folder names. And maybe warn the user to decide what to 
do with the missing unmatched folders.

Regards.