* libgimpwidgets/gimpzoommodel.[ch]: New object to handle zoom
  factor, and create widgets to control or view this value.

* libgimpwidgets/Makefile.am
* libgimpwidgets/gimpwidgets.h: Added gimpzoommodel.[ch].

* libgimpwidgets/gimpwidgetsenums.h: new enum: GimpZoomWidgetType.
  This enumerate which knid of widget you can created from the zoom
  model.

* libgimpwidgets/gimpwidgetsenums.c
* libgimpwidgets/gimpwidgetstypes.h: regenerated.

* libgimpwidgets/gimppreview.c: move the update toggle to the bottom
  of the vbox.

* libgimp/Makefile.am
* libgimp/gimpui.h
* libgimp/gimpuitypes.h
* libgimp/gimpzoompreview.[ch]: New widget, derivated from
  GimpScrolledPreview, which offer the same functionnalities as the
  GimpAspectPreview widget plus zoom facilities.

* plug-ins/common/AlienMap2.c
* plug-ins/common/apply_lens.c
* plug-ins/common/blinds.c
* plug-ins/common/channel_mixer.c
* plug-ins/common/colorify.c
* plug-ins/common/flarefx.c
* plug-ins/common/illusion.c
* plug-ins/common/jigsaw.c
* plug-ins/common/mapcolor.c
* plug-ins/common/max_rgb.c
* plug-ins/common/nova.c
* plug-ins/common/polar.c
* plug-ins/common/retinex.c
* plug-ins/common/waves.c
* plug-ins/common/whirlpinch.c: use a GimpZoomPreview instead of a
  GimpAspectPreview.