* libgimpwidgets/gimpscrolledpreview.[ch]: new function:
  gimp_scrolled_preview_set_position () to set the position of the
  preview content wrt the drawable.  This function might be moved to a
  virtual function of GimpPreview in a near future.

* libgimp/gimpdrawablepreview.c: use this function, so a preview
  remembers its position from one invocation to the next.  This is done
  using the current executable name as a key for gimp_set/get_data () so
  each plug-in can have its preview on a different place.

  More informations could be saved this way: the update toggle, the checks, etc.

  Addresses bug #162286.

* libgimpwidgets/gimppreview.c: smallish cleanup.