Change i18n of filename formatting options

Summary: Change to use ki18n to avoid duplicated context. After a suggestion in D19914#435402.

Reviewers: #spectacle, aspotashev, #localization, aacid, ngraham

Reviewed By: #spectacle, aspotashev, ngraham

Subscribers: #spectacle

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D19968