Aqbanking SEPA transfer: Really test and finish the implementation.

Also, apparently the iTAN authentication method doesn't work unless
one activates the GTK2_GUI code in gnc-gwen-gui.c:70 but I need to check
this further.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22816 57a11ea4-9604-0410-9ed3-97b8803252fd