[kcm] Fix i18n output file name and split texts

Summary:
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

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23783