Artwork (GUI+doc): CantellatedCube, Platonic polyhedra, HorizontalCylinder

To treat CantellatedCube, PlatonicOctahedron, PlatonicTetrahedron and HorizontalCylinder on par with other shapes, we need the blue 3d view and grey 2d cuts. For Dodecahedron and Icosahedron we need 2d cuts.

Shall be done automatically, once we have Img3D exposed to Python #349.

TODO: CantellatedCube:

• GUI icon
• blue 3d view
• grey 2d cuts

PlatonicOctahedron

• GUI icon
• blue 3d view
• grey 2d cuts

PlatonicTetrahedron

• GUI icon
• blue 3d view
• grey 2d cuts

Dodecahedron

• grey 2d cuts

Icosahedron

• grey 2d cuts

HorizontalCylinder

• GUI icon
• blue 3d view
• grey 2d cuts

Ellipsoid

• GUI icon

EllipsoidalSegment

• GUI icon