Cleanup for image annotations always-onish mode

Before this commit, every image annotations were always visible and only the color of the selection told that the annotation is active or not.
New behaviour:
* When the mouse cursor is over the image, all annotations are shown, and if a cursor is above a selection it gets highlighted
* Otherwise the annotations are not shown by default
* When the cursor is over the an annotation card then only that annotation's highlight (selection) is shown in the image
* Clicking on/hovering over a tab shows only tha annotation being in that bucket in the images.

Fixes #921