Grayscale previews, to help artist learn values/contrast

The idea is to have the option to change the preview feature of Aseprite into grayscale view, allowing you to view the values of your sprite in real time. This would be really helpful for people trying to improve their values! :clock1230:

while a feature like that in the preview would be cool, you also can just create a layer on top of all, fill it with black or white and put on the color layer mode to have the same effect.