D.C.’s best arts and arts organizations were honored Monday night at the 20th annual Mayor’s Arts Awards. The gala ceremony, which took place at the Kennedy Center, was presented by the D.C. Commission on the Arts & Humanities and hosted by Mayor Anthony Williams and WRC anchor Jim Vance. Leonard Slatkin (shown at left), music director of the National Symphony Orchestra, was presented the Lifetime Achievement Award in recognition of his imaginative musical programming and…