utf8: implement utf8foldernames option

If utf8foldernames is enabled on account level all folder names read
from the IMAP server will immediately be reencoded to UTF-8. Names
will be treated as UTF-8 as long as the IMAP server isn't contacted again,
for which they are reencoded to IMAP4-UTF-7.

This means that any further processing such as nametrans, folderfilter
etc. will act upon the UTF-8 names, which will have to be documented
carefully.

NOTE 1:
GMail repositories and folders inherit from the IMAP... classes, so I don't
know yet if these changes have ugly side-effects. But web research suggests
that GMail IMAP folders are equally encoded in UTF-7 so that should work
identically here and incorporate the same improvements.

NOTE 2:
I could not test the behaviour with idlefolders as I didn't get this option
to work at all, not even with the latest stable version.

NOTE 3:
I *did* test to sync an IMAP repository against another IMAP repository.

Signed-off-by: Urs Liska <git@ursliska.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/Base.py

@@ -242,7 +242,7 @@ class BaseRepository(CustomConfig.ConfigHelperMixin):
                 # Get IMAPFolder and see if the reverse nametrans works fine.
                 # TODO: getfolder() works only because we succeed in getting
                 # inexisting folders which I would like to change. Take care!
-                tmp_remotefolder = remote_repo.getfolder(remote_name)
+                tmp_remotefolder = remote_repo.getfolder(remote_name, decode=False)
                 loop_name = tmp_remotefolder.getvisiblename().replace(
                     remote_repo.getsep(), local_repo.getsep())
                 if local_name != loop_name: