DALLAS — Chief executive Gary Kelly says Southwest Airlines wasn’t even thinking about Paine Field near Seattle a couple years ago. Now it’s the third airline to announce plans to serve the airport in the northern suburbs. Kelly said Thursday that Southwest wants to increase its presence in the Seattle area but can’t grow any more at Seattle-Tacoma International Airport 45 miles away. So the Dallas-based carrier is planning to start five daily departures at Paine Field this fall.