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>

docs/MANUAL.rst

@@ -429,6 +429,41 @@ This is an example of a setup where "TheOtherImap" requires all folders to be un
     remoteuser = XXX
     #Do not use nametrans here.
 
+
+Sync from Gmail to a local Maildir with labels
+----------------------------------------------
+
+This is an example of a setup where GMail gets synced with a local Maildir.
+It also keeps track of GMail labels, that get embedded into the messages
+under the header X-Keywords (or whatever labelsheader is set to), and syncs
+them back and forth the same way as flags.
+
+The first time it runs on a large repository may take some time as the labels
+are read / embedded on every message.  Afterwards local label changes are detected
+using modification times (much faster)::
+
+    [Account Gmail-mine]
+    localrepository = Gmaillocal-mine
+    remoterepository = Gmailserver-mine
+    # Need this to be able to sync labels
+    status_backend = sqlite
+    synclabels = yes
+    # This header is where labels go.  Usually you will be fine
+    # with default value, but in case you want it different,
+    # here we go:
+    labelsheader = X-GMail-Keywords
+
+    [Repository Gmailserver-mine]
+    #This is the remote repository
+    type = Gmail
+    remotepass = XXX
+    remoteuser = XXX
+
+    [Repository Gmaillocal-mine]
+    #This is the 'local' repository
+    type = GmailMaildir
+
+
 Selecting only a few folders to sync
 ------------------------------------
 Add this to the remote gmail repository section to only sync mails which are in a certain folder::