Make GmailFolder sync GMail labels

When synclabels config flag is set to "yes" for the GMail repo,
offlineimap fetches the message labels along with the messages, and
embeds them into the body under the header X-Keywords (or whatever
'labelsheader' was set to), as a comma separated list.

It also adds an extra pass to savemessageto, that performs label
synchronization on existing messages from GMail to local, the same way
it is done with flags.

We also introduce GmailMaildir repository that adds functionality to
change message labels.  It keeps track of messages modification time,
so one can quickly detect when the labels may have changed.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/repository/Maildir.py

@@ -177,10 +177,9 @@ class MaildirRepository(BaseRepository):
                 self.debug("  This is maildir folder '%s'." % foldername)
                 if self.getconfboolean('restoreatime', False):
                     self._append_folder_atimes(foldername)
-                retval.append(folder.Maildir.MaildirFolder(self.root,
-                                                           foldername,
-                                                           self.getsep(),
-                                                           self))
+                fd = self.getfoldertype()(self.root, foldername,
+                                          self.getsep(), self)
+                retval.append(fd)
 
             if self.getsep() == '/' and dirname != '':
                 # Recursively check sub-directories for folders too.
@@ -194,6 +193,9 @@ class MaildirRepository(BaseRepository):
             self.folders = self._getfolders_scandir(self.root)
         return self.folders
 
+    def getfoldertype(self):
+        return folder.Maildir.MaildirFolder
+
     def forgetfolders(self):
         """Forgets the cached list of folders, if any.  Useful to run
         after a sync run."""