SpinButton: Update :hover on widget from gadgets

Just as we treat hovering either icon in the parent Entry as needing to
set PRELIGHT on the whole widget, do the same for the up/down buttons.

#196
!122 (comment 99084)
1 job for wip/dboles/entry-hover in 7 minutes and 1 second (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #25910
build

00:07:01