trigger: add "${tg_displayed}" in conditions of default trigger "beep"

The command of "beep" trigger is now executed only if the message is
displayed (not filtered with /filter).

src/plugins/trigger/trigger-config.c

@@ -53,7 +53,7 @@ char *trigger_config_default_list[][1 + TRIGGER_NUM_OPTIONS] =
     { "beep", "on",
       "print",
       "",
-      "${tg_highlight} || ${tg_msg_pv}",
+      "${tg_displayed) && (${tg_highlight} || ${tg_msg_pv})",
       "",
       "/print -beep",
       "ok" },