Make peak search run as a work function, so that xdatplot doesn't
block while it's going on.    

Make the cancel button of the corresponding dialog box interrupt and
break off the background action.

If region exists, marks filter should operate only on marks in region.

Write a converter from String to XmStringTable if necessary, so that
initial list of marks filters can be specified as a resource.

Add keystrokes to find next max, find next min, fit height.

Make more robust generally to installation problems: lack of expected
programs, etc.  Perhaps it should look for mosaic, xmgr, etc, and use
what it finds to create an X resource file that points to the right
place.
