By Denny Dyroff, Entertainment Editor, The Times Longwood Gardens in winter. Visitors to Longwood Gardens (Route 1, Kennett Square, 610-388-1000, www.longwoodgardens.org) during the next two months might find themselves singing lyrics from a song thought of as a Christmas carol but is really a lot more – lyrics such as… “In the lane, snow is glistenin’,  A beautiful sight, We’re happy tonight,  Walkin’...