function: write_point -- save 3D location
keyboard equivalent: <none>

Last selected 3D location (left-click) is written
to the file: $SUBJECTS_DIR/<subj>/tmp/edit.dat.
Other programs can read this file to locate the
same point.  This button also messages other
copies of tkmedit, tkregister, and tksurfer to go
to the same (tksurfer: nearest) point.  For
messaging to work, deny access to all w/xhost.
