Add option to quit when clicking close button

Current behaviour: When I click the "minimize" button, Morgen minimizes to the task bar. When I click "close", Morgen minimizes to Windows' system tray.

Requested behaviour: The behaviour of the "minimize" button remains the same. However, when I click "close", Morgen quits.

I understand that some users prefer the current behaviour. I, personally, don't need two variants of minimizing Morgen. Therefore, I suggest adding the option to choose the behaviour of the "close" button - a "Quit on close" option.