Patch #6427. Reviewed by John Darrington.

* comments-dialog.c: Adapt to new syntax generating code in
ui/syntax-gen.[ch].

* data-editor.c: Ditto.

* recode-dialog.c: Ditto.

* t-test-independent-samples-dialog.c: Ditto.

* automake.mk: Add new files.

* syntax-gen.c: New file.  Based on former
src/libpspp/syntax-gen.c but rewritten.

* syntax-gen.h: New file.  Based on former
src/libpspp/syntax-gen.h but rewritten.

* automake.mk: Remove moved files.

* syntax-gen.c: Moved to src/ui (and rewritten).

* syntax-gen.h: Ditto.