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In newer version of plasma, power-related buttons and lock button will be removed from desktop right-click-menu by default.

I enable it again, but maybe is better approach.

Add power and lock button to Active Window Control plasmoid. It could be shown, where there is no window other than desktop/panels window, or where no other window is active. It makes sense, because power/shutdown/lockscreen is related to desktop, so when plasma window is active, it may shown.

Also, it allow to save space, because user do not have to always put power/lock screen buttons on panel/desktop.