I suggest a simple dropdown: in Settings -> Window Manager -> Advanced --- below 'Double click action' label: 'Middle click action' containing: Lower (default), Minimize, Maximize, Close, Nothing I think it would fit perfectly there and is something people were asking about from what I found out during my googles. It's a really convenient feature IMO.
I am not a bug fan of adding options for everything and anything, xfwm4 already has way too many options for my taste. http://www.islinuxaboutchoice.com/
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