Wish: Improve anchor parameter

At the moment the url allows for `?anchor=topcenter` to make sure cropping behaves as we need it to when the image is auto-cropped.

But we would like to give our users much more control - but automatically. The idea is that they could mark an area on the image which is important, and the cropper would use this to limit what must stay inside the result, and what can be cropped.

Something like `?anchor=bottomright&focus=22-67,50-90`

The core idea is that the content-editor could select a rectangle on the image determining what's really important - and therefor must stay in the image no matter how we crop it. The example above would have been in %, so the important bits of the above example would be:
1. Within the width from 22% to 67%
2. Within the height of 50% to 90%

Based on this, and also because bottom-right is the anchor, the image would try to crop ensuring that this area will be visible, and expand to top-left from the 67%/90% corner.

Of course it would need to do various corrections if the image doesn't fit.

We believe this would be super-useful for many CMS scenarios, and would also help write the code if you see this is a useful addition.

Note: I noticed this is somehow similar to what the `gravity` in the JSON configuration would do. Maybe it would also be enough to allow a gravity-parameter in the url-syntax?

Start by tracing how the URL’s `anchor` parameter and the JSON `gravity` configuration currently affect auto-cropping; the issue names no source files or tests. Clarify the intended percentage-based `focus` rectangle, correction behavior when it does not fit, and what tests should demonstrate that the selected area remains visible.

Written by the indexing model from the issue text.