Add label to the lockscreen config's "Show media controls" checkbox and make QML imports consistent

Summary:

Test Plan: Open the Appearance tab of the Screen Locking KCM

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, VDG, plasma-devel, Plasma

Tags: Plasma

Maniphest Tasks: T10862

Differential Revision: https://phabricator.kde.org/D22940