so it is launchable by dropping any file on its icon. This is
done both for the configure/make and the CMake build systems
(the Xcode build system did that already).
The editor demo is the only one calling fl_open_callback().
git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@10981 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
