Keyboard shortcut for entering fullscreen

Created by: fatlazycat

Add keyboard shortcut for entering/exiting fullscreen

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Relevant code is here (MR welcome)

• F11 toggleFullscreen(), http://docs.nwjs.io/en/latest/References/Window/#wintogglefullscreen
• Esc leaveFullscreen(), http://docs.nwjs.io/en/latest/For%20Users/FAQ/#how-to-leave-fullscreen-mode-with-esc-key