Alt+F4 is closing the Greeter's window

Originally created by @spriver on #12462 (Redmine)

In 3.0 versions (at least in beta4) the Greeter’s window will get closed by pressing Alt+F4. The user will be left with a blank, blue screen (with the title bar at the top of the screen).
The keybinding Alt+F4 should be deactivated, if possible.

Feature Branch: bugfix/greeter-fixes-for-3.0,greeter:bugfix/12462-disable-alt-f4

Parent Task: #8230 (closed)

Edited May 15, 2020 by spriver

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