Set class name on status bar

This is useful for configuring compositors to ignore the status bar
window.

dwm.c

@@ -1809,6 +1809,7 @@ updatebars(void)
 		.background_pixmap = ParentRelative,
 		.event_mask = ButtonPressMask|ExposureMask
 	};
+	XClassHint ch = {"dwm", "dwm"};
 	for (m = mons; m; m = m->next) {
 		if (m->barwin)
 			continue;
@@ -1817,6 +1818,7 @@ updatebars(void)
 				CWOverrideRedirect|CWBackPixmap|CWEventMask, &wa);
 		XDefineCursor(dpy, m->barwin, cursor[CurNormal]->cursor);
 		XMapRaised(dpy, m->barwin);
+		XSetClassHint(dpy, m->barwin, &ch);
 	}
 }