Adds pages and content from Gtk.Builder
Example
using Gtk 4.0;
using Tuner 1;
translation-domain "my-translations";
Tuner.PanelPage {
tag: "my-page-tag";
title: _("Translatable title");
...
}
Tuner.PanelPageContent {
tag: "appearance"; // Add content to page with tag appearance
...
}