Synthesis andin vitrostudy of novel bis-[3-(2-arylmethylidenimino-1,3-thiazol-4-yl)-4-hydroxy-2H-chromen-2-one-6-yl]methane and bis-[3-(2-arylidenhydrazo-1,3-thiazol-4-yl)-4-hydroxy-2H-chromen-2-one-6-yl]methane as potential antimicrobial agents