forward declarations patch

[Angus Leeming]

Attached is a small patch to FormPreferences and FormPrint, moving
#include "form_xxx.h"
out of the .h files and into the .C files, replacing them with forward
declarations of the relevant structs.

All files in the src/frontends/xforms/ directory are now self consistent in
this regard.

Angus

patch19July2000_2.bz2