Prevent modifications on a folder level to occur in dry-run

Prevent savemessage(), and savemessageflags() to occur in dryrun mode in
all backends. Still need to protect against deletemessage().

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/folder/LocalStatus.py

@@ -111,6 +111,11 @@ class LocalStatusFolder(BaseFolder):
         return self.messagelist
 
     def savemessage(self, uid, content, flags, rtime):
+        """Writes a new message, with the specified uid.
+
+        See folder/Base for detail. Note that savemessage() does not
+        check against dryrun settings, so you need to ensure that
+        savemessage is never called in a dryrun mode."""
         if uid < 0:
             # We cannot assign a uid.
             return uid