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Oh, shit, that was exactly what I was looking for.  I figured there'd be such a button, good thing I thought to ask!  :)

... well, I tried it and it's slightly less convenient than what TLS had (in that one it just sent you to the main menu directly, in this one it reboots the entire game so I have to resize the window again), but it's still better than having to reopen the game manually!

You can swap between Fullscreen mode and Windowed mode with F4. So if you are playing in Fullscreen mode and want to reset. Press F5 and then F4.