Detect overhangs and enable fan

[AxTheB]

I want option to run fan when printing outside perimeters, even on object big enough so the standard cooling isnt used. Because when printing with fan on, overhangs and small details are prettier, but the object tend to delaminate too often.

[alranel]

Linking this to #328

[QuentinTorg]

I really would love a feature for this as well. Even if it turns on the fan for every single perimeter that is printed, regardless of whether or not it has overhang. My overhangs tend to curl if I don't use a fan, but leaving a fan on the entire print causes the corners to lift because my bed temp drops. Having an easy check box option for turning the fan on for perimeters sounds very nearly ideal, and simplifies the solution because there is no need to create an algorithm that calculates whether an overhang exists.

[alranel]

I implemented this! Also added test in 19b40ba