DES MOINES — Officials unveiled a proposed $500 million terminal at the Des Moines airport that they say is needed because of increased use. Officials presented plans Tuesday at a public hearing. They say the current terminal, built in 1948, isn’t large enough to handle increased use. The proposal includes 14 gates and a new parking garage. The terminal would overlap the existing building and curve to the northeast. That’s less expensive than an earlier plan, which placed a new terminal on the south side of the airport.