* plug-ins/gfig/gfig-dobject.h: renamed the DObject type to
  GfigObject, according to our common type naming.  This type will
  certainly become an abstract class in a near future.

* plug-ins/gfig/gfig-arc.c
* plug-ins/gfig/gfig-bezier.c
* plug-ins/gfig/gfig-bezier.h
* 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-line.c
* plug-ins/gfig/gfig-line.h
* plug-ins/gfig/gfig-poly.c
* plug-ins/gfig/gfig-poly.h
* plug-ins/gfig/gfig-spiral.c
* plug-ins/gfig/gfig-star.c
* plug-ins/gfig/gfig-types.h
* plug-ins/gfig/gfig.c
* plug-ins/gfig/gfig.h: changed accordingly.