* plug-ins/gfig/gfig-rectangle.[ch]
* plug-ins/gfig/images/stock-rectangle.png: news files to implement a
rectangle tool to gfig.
* plug-ins/gfig/gfig-stock.c
* plug-ins/gfig/gfig-stock.h
* plug-ins/gfig/Makefile.am: modified accordingly.
* plug-ins/gfig/gfig-circle.c
* plug-ins/gfig/gfig-dialog.c
* plug-ins/gfig/gfig-dobject.c
* plug-ins/gfig/gfig-ellipse.c
* plug-ins/gfig/gfig-poly.c
* plug-ins/gfig/gfig-star.c
* plug-ins/gfig/gfig-types.h
* plug-ins/gfig/gfig.c
* plug-ins/gfig/gfig.h: modified to support the new shape and
implement two new filling types: vertical and horizontal gradients.
These gradients are imho much more useful than the shapeburst one.
