Change kcm header "Screen Locking Timeouts" to "Configure Screen Locking"

Summary:
Change kcm header "Screen Locking Timeouts" to "Configure Screen
Locking" because it's not only about timeouts: you can also set the
background image, etc.

Reviewers: plasma-devel, broulik

Reviewed By: broulik

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8261