utf8 (aside): Move code for decodefoldernames

While intending *not* to change the behaviour of the existing
decodefoldernames option this commit transparently improves
the coding.
So far this worked by overriding the folder's getvisiblename() method
which reads self.visiblename from and applies the conversion on
*every* invocation of getvisiblename().
This commit does the calculation once in the IMAPFolder's __init__.

Signed-off-by: Urs Liska <git@ursliska.de>
(cherry picked from commit 662c7d069693e7b0aee906194a29f2a437007a5c)

offlineimap/folder/IMAP.py

@@ -47,6 +47,8 @@ class IMAPFolder(BaseFolder):
         name = imaputil.dequote(name)
         self.sep = imapserver.delim
         super(IMAPFolder, self).__init__(name, repository)
+        if repository.getdecodefoldernames():
+            self.visiblename = imaputil.decode_mailbox_name(self.visiblename)
         self.idle_mode = False
         self.expunge = repository.getexpunge()
         self.root = None # imapserver.root
@@ -290,13 +292,6 @@ class IMAPFolder(BaseFolder):
                     'keywords': keywords}
         self.ui.messagelistloaded(self.repository, self, self.getmessagecount())
 
-    # Interface from BaseFolder
-    def getvisiblename(self):
-        vname = super(IMAPFolder, self).getvisiblename()
-        if self.repository.getdecodefoldernames():
-            return imaputil.decode_mailbox_name(vname)
-        return vname
-
     # Interface from BaseFolder
     def getmessage(self, uid):
         """Retrieve message with UID from the IMAP server (incl body).