2-Substituted 4-, 5- and 6-[(1E)-3-oxo-3-phenylprop-1-en-1-yl]pyridazin-3(2H)-ones and 2-Substituted 4,5-bis[(1E)-3-oxo-3-phenylprop-1-en-1-yl]pyridazin-3(2H)-ones as Potent Platelet Aggregation Inhibitors: Design, Synthesis and SAR Studies