HaikuDepot: Fixed creating/updating ratings.

 * The HTTP Basic authentication never worked. BHttpRequest.SetUserName()
   and SetPassword() have no effect at all, unless there is a
   BHttpAuthentication set for the given URL on the BUrlContext. This part
   of the services API isn't very intuitive, yet.
 * UpdateUserRating() forgot to append the "filter" array which specifies
   to the web app which fields of the rating are updated by the request.
HaikuDepot: Changed alert message for updating a rating
HaikuDepot: Be more verbose about how to update ratings
HaikuDepot: Request to login before rating a package

 * There is now a flow of logging in or creating an account before being
   able to rate a package and the user is not yet logged in. The
   UserLoginWindow stores an "on success" BMessenger and BMessage which are
   send after the alert which confirms successful login or account creation.
   This is used to make the RatePackageWindow show once logged in successfully.