Thanks, it works.

However adding a GtkApplicationWindow and a GtkButton inside of the component tree is not updating the preview area. Adding a know valid GTK3 .ui file [0] does not update the preview area either.

Seems like the "preview" button is also missing from Glade. Oh well, let's edit XML files afterall. :)

[0] https://github.com/MicroJoe/gtk_liststore_item/blob/master/g...