Rename symbol EDITOR to DXX_USE_EDITOR to show that it is a DXX
symbol, not one inherited from a library. Move it to dxxsconf.h to
shorten the command line.
This is a mostly automated transform, but the changes to SConstruct were
manual.
git grep -wl EDITOR -- '*.h' '*.cpp' | xargs sed -i -e 's/^\s*#ifdef \(EDITOR\)\>/#if DXX_USE_\1/' -e 's/\s*#\(el\)\?if \(.*\)defined(\(EDITOR\))/#\1if \2DXX_USE_\3/' -e 's/^\s*#ifndef \(EDITOR\)\>/#if !DXX_USE_\1/'
