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/repository/LocalStatus.py

@@ -67,10 +67,11 @@ class LocalStatusRepository(BaseRepository):
 
         Empty Folder for plain backend. NoOp for sqlite backend as those
         are created on demand."""
-        # Invalidate the cache.
-        self._folders = None
         if self._backend == 'sqlite':
-            return
+            return # noop for sqlite which creates on-demand
+
+        if self.account.dryrun:
+            return # bail out in dry-run mode
 
         filename = self.getfolderfilename(foldername)
         file = open(filename + ".tmp", "wt")