AqBanking6: Found the best place for account and user management.
While starting to port AqOfxConnect to AqBanking6 I found out that this
backend needs many of the functions AqHBCI uses (and which previous
versions of AqBanking provided). So those function go into the AB_PROVIDER
module now were every backend can use them.
This patch implements the first part: Account management (reading and writing
of account configuration).