If you’ve seen the likable 2003 family movie “Secondhand Lions,” you might wonder when the mangy, old jungle cat who figures in the title will turn up in this brassy, hardworking and overstuffed musical version of the film. Now at the 5th Avenue Theatre (after weeks of previews, it runs until Oct. 6), this is the 15th new musical the company has premiered in the last decade. Will it be the ninth to reach Broadway?