Added a new icon for a "negative" volume (as of now it is the same as of a printable volume)
Added a new icon for a "negative" volume (as of now it is the same
as of a printable volume)

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r