HaikuDepot: Limit language selection for account creation

For the time being, there is a limited amount of supported languages
when creating accounts, reflect this in the UI and use a BMenuField.
Creating accounts with other languages fails, the error reporting was
not ideal and could be improved. The server responded with "object not found",
which was reported in the alert, but the response actually contains the
object which was not found (NaturalLanguageCode).
HaikuDepot: Restore logged in user across sessions.

 * Store the username in the app settings. The password was already stored
   in the keyring. Restore the username upon next launch.
 * Display the logged in user in the main window.
 * Added "Log out" menu entry.
 * When the password could not be retrieved from the keyring (also because
   the user rejects the keyring dialog), unset the username.
 * Allow unsetting the username by passing an empty name.