Do I understand correctly that there is no way to use <template> from GtkBuilder UI definition files in gtkmm? I found that in gtkmm GtkBuilder has unique method get_derived_widget that works with <object> and create object implemented by derived class. Is it added as a workaround?
Also link to RELAX NG Compact Syntax is broken on this page https://developer.gnome.org/gtk3/stable/GtkBuilder.html