Thumbnails import

Review the import sequence. 
See https://trac.openmicroscopy.org.uk/ome/ticket/10112

What to do:
 * Go to the importer
 * Select al3d folder, add it to the queue and click import
 * 2 images should be added to the importer tab
 * first one import and display
 * Second one (.xml) import and now display a black image (expected). Before the change a "message" cannot create thumbnail was displayed.