Add widget to add screenshot on gallery
add a widget to create snapshot of reconstructed slice on GALLERY folder
- for nabu slice it should be pretty simple.
- for nabu volume we should add a list of slices to be added to the gallery (like middle, one third, two third...)
To avoid keeping data in memory the better is probably to produce url (or a tuple of url) at nabu slice / volume. Tehn at widget level it will read it and move it to the gallery.