Export as OME-TIFF.

see https://trac.openmicroscopy.org.uk/ome/ticket/11940

To test
 * Select an image (not big)
    * Go to the right-hand side. 
    * Click on menu "Display Save Options"
    * Export as OME-TIFF should be not greyed out.
 * Select a big image. 
    * Go to the right-hand side. 
    * Click on menu "Display Save Options"
    * Export as OME-TIFF should be greyed out.
* Select both images
    * Go to the right-hand side. 
    * Click on menu "Display Save Options"
    * Export as OME-TIFF should be greyed out.

--rebased-from #2031