HaikuDepot: Implemented logging into haiku-depot-server

UserLoginWindow:
 * Focus nickname text field on tab switches
 * Implement testing the login info, the web-app replies with a token, if
   valid. It could be used for Token Bearer authorization of requests, but
   this is not used yet. Instead the username and password are set on the
   Model. Also after creating a new account successfully.
Model:
 * Use a member instance of WebAppInterface. Set the preferred language
   and the login-info only once.
HaikuDepot: Only authorize requests where needed.

No requests are implemented yet which need to identify a user (such as rating
a package).