set_transient_for(toplevel) was only called in list_setup(). It was easy
to make a test showing a NULL :transient-for instead of the correct one.

So, move the call from list_setup() to popup_for_device(). Also do that
for window_group_add_window(), which means not calling it redundantly.

(I tried using a ComboBox:parent-set handler, but the Inspector’s CB
didn’t like that: it calls popup_for_device() twice and closes on button
release. Anyway, using popup() is much more concise than a new handler.)