Add lock icon to desktop context menu

Summary:
Having only one icon in the whole menu seemed a waste of space.
Adding a lock icon to the lock function greatly improves the scene.

Test Plan:
Before:

After:

Reviewers: ngraham, VDG

Reviewed By: ngraham, VDG

Subscribers: abetts, safaalfulaij, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15683