* libgimpwidgets/gimppreview.c
* libgimpwidgets/gimppreview.h: split this widget into itself (more
  abstract now) and ...

* libgimpwidgets/gimpscrolledpreview.c
* libgimpwidgets/gimpscrolledpreview.h: this widget which also have
  some scrollbars and a nagivation preview.

* libgimpwidgets/Makefile.am
* libgimpwidgets/gimpwidgetstypes.h: changed accordingly.

* libgimp/gimpaspectpreview.c
* libgimp/gimpaspectpreview.h: Added this widget, derived from
  GimpPreview, which has always the same ratio has the given drawable.
  This widget has almost the same api as GimpDrawablePreview, and is
  useful for plug-ins that show the whole (scaled) drawable in their
  preview.

* libgimp/gimpdrawablepreview.c
* libgimp/gimpdrawablepreview.h: GimpDrawablePreview is now derived
  from GimpScrolledPreview.

* libgimp/Makefile.am
* libgimp/gimpui.h
* libgimp/gimpuitypes.h: changed accordingly.

* plug-ins/common/plasma.c: use a GimpAspectPreview.

* plug-ins/common/bumpmap.c
* plug-ins/common/cartoon.c
* plug-ins/common/deinterlace.c
* plug-ins/common/despeckle.c
* plug-ins/common/dog.c
* plug-ins/common/edge.c
* plug-ins/common/engrave.c
* plug-ins/common/exchange.c
* plug-ins/common/gauss.c
* plug-ins/common/grid.c
* plug-ins/common/mblur.c
* plug-ins/common/neon.c
* plug-ins/common/noisify.c
* plug-ins/common/oilify.c
* plug-ins/common/photocopy.c
* plug-ins/common/sel_gauss.c
* plug-ins/common/sharpen.c
* plug-ins/common/shift.c
* plug-ins/common/sobel.c
* plug-ins/common/softglow.c
* plug-ins/common/spread.c
* plug-ins/common/struc.c
* plug-ins/common/unsharp.c
* plug-ins/common/wind.c: use gimp_scrolled_preview_get_position
  instead of gimp_preview_get_position.