points to aqbanking should be private
free all accounts when freeing the associated collection
free the user ptr when freeing the associated collection
move validations to the head of AddPinTanUser
move supported HBCI Versions out of the func
hide gui from the outside.

the gui is an implementation detail of aqbanking, which in general
should not be of any interest to the outside
add simple banking test
add simple user tests
remove unnecessary comments
simple user.Remove() test