Fix 'Save to All' in Preview panel

This should fix the 'Save to All' bug found in #3187.

To test, edit rendering settings in Preview panel and click "Save to All". Changes should be updated in thumbnail of selected image and any other compatible images in the Dataset.

Also test that copy and paste still works OK:
 - Edit and copy settings in Preview panel.
 - Select another single image and click Paste in preview panel. Preview viewer etc should update but not thumbnail.
 - Right-click menu -> Paste and Save (with a Dataset or 1 or more Images selected). Check that thumbnails update as expected.