Download spw original files

This adds the "Download" menu (and fileset paths) to SPW images when wells are selected.

To test:
 - Import a Plate.
 - Also create a plate from a Dataset with Dataset_To_Plate.py script (simpler if images are not MIFs).
 - Select ONE well from each plate:
    - Test the "Show paths" button
    - Download the Original files for the well. This will be a single image for the created plate and the whole plate for the imported plate.
    - Export as png/jpeg/tiff. Check that you get the correct image for the current Plate 'index'.
 - Select MULTIPLE wells from each plate:
    - Test paths, download original and export png/jpeg/tiff as described above.