* devel-docs/libgimpwidgets/visual_index.xml: added this file so we
  have a "visual widget gallery" in our docs.

* devel-docs/libgimpwidgets/Makefile.am
* devel-docs/libgimpwidgets/libgimpwidgets-docs.sgml: added
  visual_index.xml.

* devel-docs/libgimpwidgets/images/gimp-*.png: placeholders for real
  widget screenshots.