for islands with another islands in their holes.
Improvement of chaining of infill lines for 3D honeycomb, Gyroid and
Honeycomb infill: New TSP chaining algorithm is used.
Changes in SupportMaterial.cpp, TriangleMesh.cpp and 01_trianglemesh.t
are yet to be merged.
WIP: Refactoring of layer height editing.
Removed layer_height_ranges from PrintObject, as the Print/PrintObject
now hold their copies of Model/ModelObject.