Maine’s largest city is betting that visitors will come to know that ‘‘life’s good here.’’ The city launched a new marketing campaign Tuesday under the slogan: ‘‘Portland, Maine. Yes. Life’s good here.’’ Besides the city, local groups and businesses plan to use the slogan, adapting it, for instance, with the sayings: ‘‘Portland. Yes. Coffee’s good here.’’ or ‘‘Portland. Yes. Printing’s good here.’’ The campaign was based on an essay written by gay rights pioneer John Preston, a native of Medford, Mass., who died in 1994.