Allow user to adjust sensibility of the UV Display Stretch heatmap

Hi everyone, sometimes we may need a nore acurate way to visualize if our UV Islands are generating stretch or not, therefore we'll want blender to tell us right away therefore break the blue and get towards red much quicker. for both efficience and visibility.

Having a stretch sensibility slider or some value somewhere near the stretch options would help a lot with that.

Thank you guys