DEVON, Pa.—One of the first welcome signs of spring is the opening of the ticket office of the Devon Horse Show and Country Fair, which will open this year on April 8.   The 117th edition of the Devon show runs from May 23 through June 2 and benefits Bryn Mawr Hospital, with proceeds to the Hospital approaching $15 million, over $50 million when adjusted for inflation.   Exhibitors and spectators will see many improvements when the show opens, including more seating for the $100,000 Wells Fargo Grand Prix on Thursday evening, which is always sold out well in advance