Implement ui.makefolder and abort repo.makefolder() in dry-run mode

IMAP, Maildir, and LocalStatus abort if in dry-run mode. IMAP and Maildir
will log that they "would have" created a new folder.

This will probably fail later on as we can not cache messagelists on
folder that don't exist, so --dry-run is not yet safe when new folders
have been created.

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

offlineimap/ui/UIBase.py

@@ -298,6 +298,12 @@ class UIBase(object):
                            (src_repo, dst_repo))
 
     ############################## Folder syncing
+    def makefolder(self, repo, foldername):
+        """Called when a folder is created"""
+        prefix = "[DRYRUN] " if self.dryrun else ""
+        self.info("{}Creating folder {}[{}]".format(
+                prefix, foldername, repo))
+
     def syncingfolder(self, srcrepos, srcfolder, destrepos, destfolder):
         """Called when a folder sync operation is started."""
         self.logger.info("Syncing %s: %s -> %s" % (srcfolder,