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- Include unpublished Dt headers in Dt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 for unpublished Motif functions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/WmMain.c

@@ -55,6 +55,7 @@ static char rcsid[] = "$TOG: WmMain.c /main/8 1998/04/20 13:01:09 mgreess $"
 #include "WmCEvent.h"
 #include "WmEvent.h"
 #include "WmInitWs.h"
+#include "WmError.h"
 #ifdef WSM
 #include "WmIPC.h"
 #include "WmBackdrop.h"