Add a lock screen action to the system menu

Was implemented in feature/jessie, and then reverted in 4d2a7b565410d007c4713936a6904494fb991cab since we decided at the December 2014 meeting that the bigger problem was more complicated (#8383 (closed)).

Feature Branch: segfault:feature/5684-screen-locker

Parent Task: #5684 (closed)

Subtasks

#11256 (closed)

Related issues

Blocked by #8383 (closed)

Original created by @tails on 5878 (Redmine)

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