Patch from Chris Shoemaker to privatize the gnucash string cache code

[David Hampton]

Log Message:
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Patch from Chris Shoemaker to privatize the gnucash string cache code
and force all usage through gnc_string_cache_{remove,insert}()
functions.  This makes debugging various uses (and misuses) of the
cache easier.

Tags:
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gnucash-gnome2-dev

Modified Files:
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    gnucash/src/engine:
        gnc-engine-util.c
        gnc-engine-util.h