1. Instead of creating a C wrapper around gettext to then wrap in
guile, use guile's builtin gettext support directly.
The code still defines the _ and N_ shorthands. However it doesn't
really warant a separate module just for these two shorthands.
Instead define them in core-utils. So all code wanting to use
_ or N_ in guile should now use the (gnucash core-utils) module.
The bulk of this commit is actually deleting the scm-gettext
target and using (gnucash core-utils) instead of (gnucash gettext).
2. As the definition of _ and N_ is removed from app-utils.scm,
the app-utils test for a functional N_ macro has been moved to a
new test file in the guile bindinds tests.
3. The (gnucash gettext) module has been deprecated. Use
(gnucash core-utils) from now on.
Note core-utils.i is used by both the guile and the python bindings so
it is moved up to the common bindings directory, while guile
specific changes are in bindings/guile.
