Save as file name

If a Batch_Image_Export.zip already exists in the destination folder, append a Batch_Image_Export_(xxx).zip where xxx is a number 

To test:
 * select an image, Save as JPEG for example (From right-hand pane)
 * select another image. Save As JPEG. Make sure the destination folder is the same in both cases.

See https://trac.openmicroscopy.org.uk/ome/ticket/10767