Add support for client-side-parsed SQL-like queries.

[Linas Vepstas]

Log Message:
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Add support for client-side-parsed SQL-like queries.  
Users can now write things such as 
'SELECT * FROM xaccTransaction WHERE datePosted > 4/4/4;"
and have the qof query mechanism work as before, and return
a glist of transactions.  For example demo usage, see the qof tarball.
Works, but needs improvement.

Added Files:
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    gnucash/src/engine:
        qofsql.c
        qofsql.h