Add manual backend

This does not:

• Remove the need for weboob
• Does not move the "add transaction" button to the operations list (this would become a real need though)
• Does not allow to modify the access name
• Implement a UI to add manual accounts
• Allow to add a bank access without login/password (just fill anything)

But I think this could already help several of our users for a really small amount of efforts.

This generates 3 default accounts (one in USD). The user can already modify the accounts titles and remove the unused accounts.