* 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.
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