Fix warnings in dtwm

- Improve pointer/int compatibility
- Include unpublished Dt headers in Dt
- Use <Xm/XmPrivate.h> for unpublished Motif functions

There are still warnings left generated because
ElementValue.parsed_value should really be a union.

There are also some warnings left because of XtPointer
casting and some unused variables and functions.

cde/programs/dtwm/Print.c

@@ -223,7 +223,7 @@ PrintFrontPanelContents(void)
  *
  ************************************************************************/
 
-void
+int
 main (int argc,
       char         **argv)