Only show "Unlock Widgets" in the UI, not "Lock Widgets"

Summary: Not needed for normal users with the new edit mode system.

Test Plan: Lock the widgets before compiling this patch. Then compile with it, unlock widgets and see that there is no lock option in the panel context menu.

Reviewers: Plasma, VDG, ngraham, mart

Reviewed By: Plasma, VDG, ngraham, mart

Subscribers: broulik, mart, ngraham, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24839