dtwm: Cleanup some implicit definitions

Mostly this is adding appropriate #includes and declarations,
but for WmImage.c we also change from using the proper name
for XmeGetMask, rather than the identical but renamed
version _DtGetMask which is not exported in any header.

cde/programs/dtwm/Print.c

@@ -42,6 +42,7 @@
 #include <Dt/DbReader.h>            /* required for DtDbPathId type */
 #include <Dt/WsmM.h>
 #include <Dt/IconFile.h>
+#include <Dt/Dt.h>
 #include "DataBaseLoad.h"
 #include "Parse.h"