Add support for alternative message date synchronisation

Global or per-repository option utime_from_message tells OfflineIMAP
to set file modification time of messages pushed from one repository
to another basing on the message's "Date" header.

This is useful if you are doing some processing/finding on your
Maildir (for example, finding messages older than 3 months),
without parsing each file/message content.

From: Cyril RUSSO <boite.pour.spam@gmail.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/Base.py

@@ -15,7 +15,7 @@
 #    along with this program; if not, write to the Free Software
 #    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 
-from offlineimap import threadutil
+from offlineimap import threadutil, emailutil
 from offlineimap import globals
 from offlineimap.ui import getglobalui
 from offlineimap.error import OfflineImapError
@@ -48,6 +48,13 @@ class BaseFolder(object):
         if self.visiblename == self.getsep():
             self.visiblename = ''
         self.config = repository.getconfig()
+        utime_from_message_global = \
+          self.config.getdefaultboolean("general",
+          "utime_from_message", False)
+        repo = "Repository " + repository.name
+        self._utime_from_message = \
+          self.config.getdefaultboolean(repo,
+          "utime_from_message", utime_from_message_global)
 
     def getname(self):
         """Returns name"""
@@ -66,6 +73,10 @@ class BaseFolder(object):
         """Should this folder be synced or is it e.g. filtered out?"""
         return self._sync_this
 
+    @property
+    def utime_from_message(self):
+        return self._utime_from_message
+
     def suggeststhreads(self):
         """Returns true if this folder suggests using threads for actions;
         false otherwise.  Probably only IMAP will return true."""
@@ -327,6 +338,9 @@ class BaseFolder(object):
             message = None
             flags = self.getmessageflags(uid)
             rtime = self.getmessagetime(uid)
+            if dstfolder.utime_from_message:
+                content = self.getmessage(uid)
+                rtime = emailutil.get_message_date(content, 'Date')
 
             if uid > 0 and dstfolder.uidexists(uid):
                 # dst has message with that UID already, only update status