HaikuDepot: Added captcha request to WebAppInterface

Also removed some disabled debug output.
HaikuDepot::Model: Set preferred language earlier

Added accessor for preferred language.
HaikuDepot: Get captcha image in UserLoginWindow
HaikuDepot: Added WebAppInterface::CreateUser()
HaikuDepot: Implemented creating web-app user accounts

The accounts are created for real, but the information is not yet stored.
Nothing which would need it is currently implemented, like rating packages.
It is recommended to create accounts with a valid email address. Otherwise
the password cannot be reset, and it will need to be soon, since there
is a small change to the password storage in the pipe.

The error response from the service is parsed and presented
to the user, when the account could not be created. However, other checks
before contacting the server are not performed, like if the two passphrases
actually match.

The UserLoginWindow now has the concept of a running worker thread, and
while it runs, the UI controls (except Cancel) are disabled.