Allow plural form for XX day forecast

Summary: Currently KMyMoney doesn't allow plural form to translate this
string. That doesn't translate well in some languages, for instance
French.

GUI:

Reviewers: tbaumgart

Reviewed By: tbaumgart

Differential Revision: https://phabricator.kde.org/D29306

(cherry picked from commit 33eca83146db16480a556c24863f17cb85114e44)