Implement adding currencies

This patch changes following:
1) only single currency is created during new .kmy creation, to prevent
currency clutter,
2) currencies can be added and deleted in currency dialog,
3) ancient currency prices are added with ancient currency.

Differential Revision: https://phabricator.kde.org/D5271



Signed-off-by: Łukasz Wojniłowicz <lukasz.wojnilowicz@gmail.com>