add sync_this variable to all Folder() instances

This variable shows if this folder should be synced or is disabled due to
a folderfilter statement. This lets us distinguish between a non-existent
folder and one that has been filtered out. Previously any filtered folder
would simply appear to be non-existing.

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

offlineimap/folder/Base.py

@@ -33,6 +33,8 @@ class BaseFolder(object):
         :para name: Path & name of folder minus root or reference
         :para repository: Repository() in which the folder is.
         """
+        self.sync_this = True
+        """Should this folder be included in syncing?"""
         self.ui = getglobalui()
         self.name = name
         self.repository = repository