Gnc-Prefs: Convert gnc user warnings

This is a little more involved that a simple
find/replace of gconf->gsettings functions.

Gsettings doesn't allow introspection on the schema
like gconf does. So we need another way to extract
the warnings' summaries and long descriptions to
show the user. To fix this, a new set of source
files is auto-generated based on the gsettings schema
source file for the warnings: gnc-warnings.[ch]

gnc-warnings.c: contains an array of warning
description structs and the implementation of
an accessor function. The array is to be assumed
read only

gnc-warnings.h: defines each warning using #define
and exposes the accessor function to the rest of the
source tree.

The two source files are constructed using an xsl
transformation on the gsettings schema source file.
This is done using the tool xsltproc, which should
already be available on each platform GnuCash supports
because it gets installed together with libxml2,
which is already a required dependency for GnuCash.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23260 57a11ea4-9604-0410-9ed3-97b8803252fd