* libgimpwidgets/gimppreview.c
* libgimpwidgets/gimppreview.h: renamed the "updated" signal to
  "invalidated" and the confusing "update" virtual function to "draw".

Renamed the property with saner names, too.

Removed _get_width and _get_height functions in favor of a _get_size on.

Added a gimp_preview_invalidate function that emit the "invalidated"
signal if needed.

* libgimp/gimpdrawablepreview.c
* libgimp/gimpdrawablepreview.h: modified accordingly and fixed the
  scrollbar range.

* 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: modified accordingly.