Milestone 2: The Grayscale button

In this simplest of all the milestones, your job is to add a Grayscale button that replaces the image with a new one in which the image has been converted to grayscale. The code necessary for this transformation appears not only in the book but also in the file GrayscaleImage.py in the repository. To implement this milestone, you should not copy and paste the code from GrayscaleImage.py, but instead import the function or functions you need. The advantage of doing so is that there is then only one copy of the code, which is shared between the two applications. Copying code inevitably creates problems for software maintenance, since changes made to one copy may not be incorporated into the other, leading to incompatible versions.