store_async
Description:
Commit the changed account settings to the account database, and invoke callback when the operation has been completed.
Parameters:
| this | 
           the Account.  | 
      
| cancellable | 
           optional Cancellable object, null to ignore.  | 
      
| callback | 
           function to be called when the settings have been written.  | 
      
| user_data | 
           pointer to user data, to be passed to   |