Better handling of read-only folders. We will now warn if there is a
change, but not propogate it. New config variable ignore-readonly can
suppress the warnings. This fixes [complete.org #10] and, for Debian,
Closes: #154769. changelog: noted the change IMAP.py: trap
imapobj.readonly more often UIBase.py: new methods to handle the
warnings offlineimap.conf: new ignore-readonly variable.