[GSOC] Data model unit testing

[Christian Stimming]

Here's the exact inside/outside result:

Am Freitag, 1. April 2011 schrieb John Ralls:
> Groups with no outside users are QofGobject

That's a header that contains only macros, and those macros are indeed in use 
throughout gnucash, e.g. QOF_GOBJECT_IMPL.

> QofSql, QofSerialize, and QofDeserialize. 

The c files haven't been compiled for quite some time by now. I'll remove them 
and the headers in a minute.

> Those with very few and which I haven't looked at in
> detail are QofReference (1)

Is used only internally in QofSession, so I turned the header into a "private" 
one.

> , QofChoice (6)

Is used throughout gnucash.

> , and Md5.

Is used in src/import-export/aqbanking/gnc-gwen-gui.c (and probably useful 
there) but nowhere else. The code can very well be moved to some other place, 
but it is in use in the (optional) aqbanking module so please make sure you 
compile with --enable-aqbanking to verify the module hasn't been broken by the 
move.

Regards,

Christian