[kcm] Fix i18n output file name and split texts
The output file name must be kcm_kscreen.po for translators to pick it up and
text being split to respect the 100 chars line limit must first be translated
before being merged together again.
Test Plan: KCM starts without errors.
Reviewers: Plasma, victorr, yurchor
Reviewed By: yurchor
Differential Revision: https://phabricator.kde.org/D23783