Initialize event_last_[xy] to out of range coordinates and also update
these values in enter and leave notify.  Fix up calls to
update_prelight() from size allocate.  Unconditionally doing these calls
caused problems with hover selection.  Now we only do this call when
the "width before the expander column" has changed.  (Which might be
awkward, but it is the best heuristic I could come up with so far).