Make rotated geometry warning in Image Cropper more consistent

Summary:
See T29093 . The warning message for images with a rotated geometry has been updated to not promise pixel-alignment after a reinit. It is also shown more consistently, by only being updated when the image selection changes (previously, adding and removing a bounding box would always make the warning disappear).
While at it, I made the node predicate of the image selection more specific, to filter out bounding shapes as possible selections.

Test Plan: Load an image with a rotated geometry (e.g. brain.nrrd). The Image Cropper plugin should always show a warning message when this image is selected. Try a different image without a rotated geometry (e.g. Pic3D.nrrd), where the message should not be visible.

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Subscribers: kislinsk

Differential Revision: https://phabricator.mitk.org/D654