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/SearchCalls.c

@@ -48,11 +48,15 @@
 #include "EditorP.h"
 #include <Xm/TextF.h>
 #include <Xm/MessageB.h>
+#include <Xm/List.h>
 #include <ctype.h>
 #include <limits.h>
 #include <Dt/DtMsgsP.h>
+#include <Dt/HourGlass.h>
 #include "DtWidgetI.h"
 
+#include <Xm/XmPrivate.h>    /* _XmStringSourceGetString */
+
 #define X_INCLUDE_STRING_H
 #define XOS_USE_XT_LOCKING
 #include <X11/Xos_r.h>