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- Include unpublished Dt headers in Dt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 for unpublished Motif functions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

@@ -1244,7 +1244,7 @@ void WmInstallBitmapDataIntoXmCache (WmScreenData *pSD,
 {
     XImage *pImage;
 
-    if (pImage = (XImage *) XtMalloc (sizeof (XImage)))
+    if ((pImage = (XImage *) XtMalloc (sizeof (XImage))))
     {
 	pImage->width = width; 
 	pImage->height = height;