add toggle to show keypoint coverage

[AlexRiina]

Add a toggle to show the keypoints that the detector has identified for more easily debugging whether the an image contains copied sections.

I was looking into an image that has some copied subsections that were not detected by sherloq and noticed that at low contrast the image has very few keypoints detected. Bumping up the contrast helped the detectors find more points but it wasn't immediately obvious what was wrong because the detectors do still detect many keypoints.

My hard-coded color was not chosen thoughtfully, but could be set to the inverse of the center point.

[GuidoBartoli]

Thanks! 👍

Tomorrow I will be on holiday for 2 weeks, but when I will come back I will test and merge your addition and let you know.

EDIT: I saw the small change, I merged on trust before leaving 😉