DtWidget: Improve portability of Motif interface

- Add missing includes and prototypes
- Improve type compatibility
- Use <Xm/XmPrivate.h> for unofficial libXm headers

With this patch, dtpad no longer crashes on startup
on FreeBSD/amd64 because of a truncated 64-bit pointer.

cde/lib/DtWidget/EditCalls.c

@@ -49,6 +49,7 @@
 
 #include "EditorP.h"
 #include "DtWidgetI.h"
+#include <Xm/XmPrivate.h>  /* _XmStringSourceGetString */
 
 void
 _DtEditorResetUndo(