Add an "Exit" item to the main menu, with keyboard shortcut

[dhmw]

Is your feature request related to a problem? Please describe.

When running this app under a window manager such as i3wm, there's no way to quit the app. I have to use xkill to terminate it.
There's no menu item to exit.
There's no standard keyboard shortcut to exit.
i3wm does not (by default) provide (x) button on the title bar to close the application.

Describe the solution you'd like

Add a menu item and standard keyboard shortcut to exit the application; I would suggest CTRL-Q.

Describe alternatives you've considered

I have to use xkill to forcibly quit the app.