Apply patch from Neil Williams <linux@codehelp.co.uk> Added

[Linas Vepstas]

Log Message:
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Apply patch from Neil Williams <linux at codehelp.co.uk>
Added QofSetterFunc to Account, Transaction and gnc_pricedb to aid
gncBookMerge and SQL backend

Modified Files:
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    gnucash/src/engine:
        Account.c
        Transaction.c
        gnc-pricedb.c