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- Include unpublished Dt headers in Dt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 for unpublished Motif functions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/DtSvcInternal.h

@@ -0,0 +1,2 @@
+/* Additional import */
+extern Pixmap _DtGetMask(Screen *screen, char *image_name);