Add keyboard shortcut for shutdown dialog

If you want to add a custom keyboard shortcut to open the shutdown menu, you have to:

• go on Menu --> Keyboard, and then on the "Shortcuts" tab
• click on the "+" button
• give a name you desire and the following command:  gnome-session-quit --power-off
• Select the new shortcut and press the key combination you want to assign to it
• Close, and you're done!

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