* libgimpwidgets/gimppreview.c
* libgimpwidgets/gimppreview.h: added a GimpPreview widget, abstract
  base for a GimpDrawablePreview.

* libgimpwidgets/Makefile.am
* libgimpwidgets/gimpwidgets.h
* libgimpwidgets/gimpwidgetstypes.h: modified accordingly.

* libgimp/gimpdrawablepreview.c
* libgimp/gimpdrawablepreview.h: added a GimpDrawablePreview widget
  to ease	the use of previews by plug-ins.

* libgimp/Makefile.am
* libgimp/gimpui.h: Changed accordingly.

* plug-ins/common/despeckle.c
* plug-ins/common/gauss.c
* plug-ins/common/neon.c
* plug-ins/common/sobel.c
* plug-ins/common/softglow.c
* plug-ins/common/spread.c
* plug-ins/common/unsharp.c: use a GimpDrawablePreview with these plug-ins.