Fix relative positioning between two new elements in gprefs().

Fix relative positioning between two new elements in gprefs().

It was only prepared to position a new element relative to an existing
one if the latter was specified in the saved configuration, not if the
latter was another element new to this run of gprefs(). This wasn't
deliberate at all: it was just due to me failing to update the 'seen'
bitmap in the loop adding new elements.