Implement setting the WM_NAME of i3 container windows for debugging

src/manage.c

@@ -185,6 +185,10 @@ void manage_window(xcb_window_t window, xcb_get_window_attributes_cookie_t cooki
     nc->window = cwindow;
     x_reinit(nc);
 
+    char *name;
+    asprintf(&name, "[i3 con] container around %p", cwindow);
+    x_set_name(nc, name);
+    free(name);
 
     /* set floating if necessary */
     bool want_floating = false;