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/Callback.c

@@ -58,6 +58,7 @@
 #include <Xm/Form.h>
 #include <Xm/ToggleBG.h>
 #include <Xm/AtomMgr.h>
+#include <Xm/TextF.h>
 
 #include <X11/Xatom.h>
 #include <X11/keysym.h>