Set accountname in BaseFolder, and don't pass it in initialization

We passed in the accountname to all derivatives of BaseFolder, such as
IMAPFolder(...,repository,...,accountname), although it is perfectly
possible to get the accountname from the Repository(). So remove this
unneeded parameter. Each backend had to define getaccountname() (although
the function is hardly used and most accessed .accountname directly).

On the other hand BaseFolder was using getaccountname but it never defined
the function. So make the sane thing, remove all definitions from backends
and define accountname() once in Basefolder. It was made a property and not
just a (public) attribute, so it will show up in our developer
documentation as public API.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Maildir.py

@@ -58,14 +58,13 @@ def gettimeseq():
         timelock.release()
 
 class MaildirFolder(BaseFolder):
-    def __init__(self, root, name, sep, repository, accountname, config):
+    def __init__(self, root, name, sep, repository, config):
         super(MaildirFolder, self).__init__(name, repository)
         self.config = config
         self.dofsync = config.getdefaultboolean("general", "fsync", True)
         self.root = root
         self.sep = sep
         self.messagelist = None
-        self.accountname = accountname
 
         self.wincompatible = self.config.getdefaultboolean(
             "Account "+self.accountname, "maildir-windows-compatible", False)
@@ -80,9 +79,6 @@ class MaildirFolder(BaseFolder):
         # Cache the full folder path, as we use getfullname() very often
         self._fullname = os.path.join(self.getroot(), self.getname())
 
-    def getaccountname(self):
-        return self.accountname
-
     def getfullname(self):
         """Return the absolute file path to the Maildir folder (sans cur|new)"""
         return self._fullname