NEW YORK — Käthe Kollwitz is a persistent presence throughout the Museum of Modern Art retrospective of her work. She confronts visitors at the opening of the show, in self-portraits made early in her career, staring at the viewer with youthful self-possession and a keen sense of intelligence. And she is seen at the end of the exhibition, as an elderly figure in the late 1930s, in yet more self-portraits and as the unnamed woman in a lithograph, “Call of Death,” with a spectral hand touching her shoulder.