New trace subsystem

[Neil Williams]

Log Message:
-----------
New trace subsystem

Tags:
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gnucash-gnome2-dev

Modified Files:
--------------
    gnucash/src/engine:
        gnc-engine.c
        gnc-engine.h
        gnc-trace.h
        gnc-trace.c
        qofbook.h
        qofbook.c
        qofquery.c
        qofsession.c
        gnc-pricedb.c