* libgimpwidgets/gimpwidgetsenums.h: remove the GIMP_ZOOM_LABEL value
  from GimpZoomWidgetType.

* libgimpwidgets/gimpwidgetsenums.c: regenerated.

* libgimp/gimpzoompreview.c: provides two read only properties,
  "fraction" and "percentage" to be used with prop widgets.

* libgimpwidgets/gimpzoommodel.c: use a gimp_prop_label to show the
  zoom fraction.