Place chicken in 3-1/2 to 4-quart slow cooker. Top with onion, olives and capers. Sprinkle with oregano, salt, rosemary and garlic powder. Pour tomatoes over chicken. Cover; cook on low setting for 7 to 10 hours or until chicken is fork-tender, its juices run clear and onion is tender. Remove chicken and vegetables from slow cooker with slotted spoon; place on serving platter. Cover to warm. In small bowl, combine water and cornstarch; blend well. Add to slow cooker. Increase setting to high; cook until thickened. Serve with chicken.