[sylpheed:33714] Better support of Junk handling in the UI

[Michael A. Puls II]

Sylpheed 3.0.0 - WinXP - IMAP

If you specify an IMAP mailbox as a Junk folder in 'Configuration ->
Common Preferences -> Junk mail', when right-clicking on a mailbox and
going to properties, there should be a new type named 'Junk' (with a junk
icon) so that the mailbox can act like a Junk folder where you can
right-click on it and empty it.

Also, specifying a Junk mailbox shouldn't be a common preference. It
should be per account and be under the advanced tab in the account's
properties in the same spot that you specify sent, drafts and trash
mailboxes etc.

-- 
Michael