Add basic image comparison functionality

[beatngu13]

Problem

Based on this discussion, I believe it would be helpful to have a very basic image comparison functionality. Many GUIs such as websites have e.g. icons or image banners. Currently, we aren't notified when such content changes—which is not ideal for a lib that is said to support visual testing.

Solution

As suggested by @roesslerj, a very basic implementation could try to extract hashes for images and then these hashes can be compared.

Also, the question arises if we can remove the legacy stuff from de.retest.recheck.image.