Development of a Highly Potent D2/D3 Agonist and a Partial Agonist from Structure–Activity Relationship Study of N6-(2-(4-(1H-Indol-5-yl)piperazin-1-yl)ethyl)-N6-propyl-4,5,6,7-tetrahydrobenzo[d]thiazole-2,6-diamine Analogues: Implication in the Treatment of Parkinson’s Disease