gtk-demo: Use GtkBuilderScope more
Under circumstances I haven't fully tracked down, these demos refuse to run, failing to locate their callbacks. So use the machinery we have, and set up a GtkBuilderCScope for each of the problematic cases.
Under circumstances I haven't fully tracked down, these demos refuse to run, failing to locate their callbacks. So use the machinery we have, and set up a GtkBuilderCScope for each of the problematic cases.