The Texas cattle-country town of Granbury (pop.: 5,718) is an ideal spot for weekend getaways. Located about 65 miles southwest of Dallas, it boasts a stone opera house built in 1886, a double-decker riverboat, a well-worn Jesse James legendeverything except regular airline service. But if an air-taxi demonstration in Danville, Virginia, this month goes as planned, tourists could soon be zipping in and out of Granbury as though it were Dallas.