accounts.py: Merge AccountSynchronizationMixin with SyncableAccount class

AccountSynchronizationMixin was never used on its own and it is a very
confusing class until you understand what it is used for. (It
complemented the Account() class with a few methods to make Account()
syncable.

But we use the SyncableAccount class anyway, so merge the former Mixin'
methods directly in there.

This does away with a class that is not directly used, and was a case of
over-object-orientation which confuses more than it helps.

Touched up code documentation while going through the file.

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

offlineimap/accounts.py

@@ -166,7 +166,12 @@ class Account(CustomConfig.ConfigHelperMixin):
             item.stopkeepalive()
         return sleepresult
             
-class AccountSynchronizationMixin:
+    
+class SyncableAccount(Account):
+    """A syncable IMAP account.
+
+    Derives from class:`Account`."""
+
     def syncrunner(self, siglistener):
         self.ui.registerthread(self.name)
         self.ui.acct(self.name)
@@ -286,11 +291,11 @@ class AccountSynchronizationMixin:
         except:
             self.ui.warn("Exception occured while calling hook")
 
-class SyncableAccount(Account, AccountSynchronizationMixin):
-    pass
 
 def syncfolder(accountname, remoterepos, remotefolder, localrepos,
                statusrepos, quick):
+    """This function is called as target for the
+    InstanceLimitedThread invokation in SyncableAccount."""
     global mailboxes
     ui = getglobalui()
     ui.registerthread(accountname)