modules: move 'content' and 'anchors' to MODULE_COMMON_ATTRS

All modules implement these. The only one that differs is the i3
module. It still implements them, but 'content' uses an internal
verifier function.

modules/xwindow.c

@@ -346,8 +346,6 @@ static bool
 verify_conf(keychain_t *chain, const struct yml_node *node)
 {
     static const struct attr_info attrs[] = {
-        {"content", true, &conf_verify_particle},
-        {"anchors", false, NULL},
         MODULE_COMMON_ATTRS,
     };