Get the Aqbanking transfer action to work again: Revert part of 7faed6540162ccd1fe.

As discussed in
https://lists.gnucash.org/pipermail/gnucash-devel/2014-July/037868.html
as an exception the direct access to the kvp is enabled again in
import-export/aqb/gnc-ab-kvp.c because the way aqbanking transfer templates
are stored cannot be mapped directly on the properties mechanism.
With this implementation, at least the read access to the templates
works again, which means the aqbanking transfers can be used again (they
crashed immediately up until now).

However, writing a changed template to the kvp seems to not work - the
changed values seem not to be saved. This would be a bug, but not as severe
as the previous one. The unittest verifies both reading and writing,
though, and there also writing works fine. Well, this can be worked
on later.