Import delete dataset

Fix issue reported by https://trac.openmicroscopy.org.uk/ome/ticket/11430

To test:

Test 1:
 * Create a dataset: toDeleteBeforeImport from Data manager
 * Open the importer
 * Select an image
 * Select the dataset created above.
 * Add the image to the import queue.
 * Go to web or data manager, delete the dataset
 * click import
 * The image should be imported, and be orphaned.

Test 2:
 * Create a project: toDeleteBeforeImport from Data manager
 * Open the importer
 * Select a folder
 * Select the project created above.
 * Add the folder to the import queue.
 * Go to web or data manager, delete the project
 * click import
 * The folder should be imported, and be a dataset w/o project

Test 3:
 * Create a screen: toDeleteBeforeImport from Data manager
 * Open the importer
 * Select a plate
 * Select the screen created above.
 * Add the plate to the import queue.
 * Go to web or data manager, delete the plate
 * click import
 * The plate should be imported, and be orphaned.