in Makefile:
 - build invalidate.o, geometry.o and events.o                (when ready)

in events.c, invalidate.c, geometry.c:
 - adapt to the new regioning code

for all of these *.c:
  - finish the adaptation to the ggiNamingScheme
  - Test visible region code and implement invalid regions !! Hint: use
    visible regions before/after a move/resize/show/hide (this
    will give an unoptimised, but simple gwt - then we optimise)


in demos/ :
 - implement some test programs:
    * test of basic creation/destruction, root attachment
    * test of zorder and part-owner hierarchy
    * test of invalid region and windows position/size       (when ready)
    * test of event dispatching system                       (when ready)

