Prepare for wrapping gnc-hooks APIs.

[Derek Atkins]

Log Message:
-----------
Prepare for wrapping gnc-hooks APIs.

	* src/engine/gw-engine-spec.scm:
	  use gnc-hooks-scm.h header
	* src/engine/gnc-hooks.[ch]:
	  Fix APIs..  Even though we return 'const' don't say we do.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/engine:
        gnc-hooks.c
        gnc-hooks.h
        gw-engine-spec.scm