Add /titlebar show presence command

Additionally to `/presence titlebar on` we now allow the setting via
/titlebar show|hide presence` since it's about the titlebar.

Regards https://github.com/profanity-im/profanity/issues/1116

src/command/cmd_ac.c

@@ -777,6 +777,7 @@ cmd_ac_init(void)
     autocomplete_add(titlebar_show_ac, "tls");
     autocomplete_add(titlebar_show_ac, "encwarn");
     autocomplete_add(titlebar_show_ac, "resource");
+    autocomplete_add(titlebar_show_ac, "presence");
 
     tls_certpath_ac = autocomplete_new();
     autocomplete_add(tls_certpath_ac, "set");