by @supermerill and @wavexx
WIP: The function Fill::connect_infill() is being rewritten
to utilize spatial structures wherever possible for lower CPU load
and less dynamic memory allocation.
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.