Fix older adwaita tab bars appearing above the title bar

[danudey]

When setting up the GTK window with older Adwaita versions, we need to explicitly create and add a tab bar. The previous code simply prepended the tab bar into the GtkBox, which resulted in it being the very first element (ahead of the title bar).

Instead, we grab a reference to the GTK title bar widget and call gtk_box_insert_child_after() to explicitly insert our tab bar into the hierarchy just after the title bar.