Improvements to the SQlite-based local status folder

* Do not inherit LocalStatusSQLiteFolder class from the plaintext
    one.

  * Use some functions already in BaseFolder in both, plaintext and
    sqlite classes.

  * Add a saveall method. The idea is that saveall dumps the entire
    messagelist to disk, while save only commits the uncommited
    changes. Right now, save is noop for sqlite, and equivalent to
    saveall for plaintext, but it enables to be more clever on when we
    commit to disk in the future.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/Base.py

@@ -36,6 +36,7 @@ class BaseFolder(object):
         # Save original name for folderfilter operations
         self.ffilter_name = name
         # Top level dir name is always ''
+        self.root = None
         self.name = name if not name == self.getsep() else ''
         self.repository = repository
         self.visiblename = repository.nametrans(name)