Add /titlebar show resource command

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

But makes sense to have it in `/resource` too because there is
`/resource message on|off` too. And this one doesnt have an setting of
it's own.

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

src/command/cmd_ac.c

@@ -776,6 +776,7 @@ cmd_ac_init(void)
     titlebar_show_ac = autocomplete_new();
     autocomplete_add(titlebar_show_ac, "tls");
     autocomplete_add(titlebar_show_ac, "encwarn");
+    autocomplete_add(titlebar_show_ac, "resource");
 
     tls_certpath_ac = autocomplete_new();
     autocomplete_add(tls_certpath_ac, "set");