Add support for saving and loading tax account mappings to and from a template file

Summary:
A tax account mapping is stored in an xml tag of type 'flag' with the attribute
name='VatAccount'. The attribute 'value' contains a reference to the corresponding
tax account, which contains the reference in the attribute 'id'.

When importing a template file with a tax account assignment, the reference is
first stored in a temporary key/value pair with the name 'UnresolvedVatAccount'
of the account concerned and after completion of the load is stored in the
key/value pair 'VatAccount' based on the ID of the tax account now available.

CCBUG:380286

Reviewers: tbaumgart, wojnilowicz

Differential Revision: https://phabricator.kde.org/D16726