* libgimpwidgets/gimpwidgetsmarshal.list: added VOID__DOUBLE_DOUBLE
 marshaller.

* libgimpwidgets/gimpzoommodel.[ch]: new signal: "zoomed", emitted
  when the zoom factor changes.  This signal have the old factor and the
  new factor as argument.

* libgimp/gimpzoompreview.c: use the "zoomed" signal instead of the
  "notify::value" one. This allow to use the old and new factors to keep
  the preview centered on the same point.