so that the ordering code at G-code export may work directly with
pointers to PrintInstances instead of with pair of <PrintObject, copy idx>.
Also the PrintInstance knows its source ModelInstance, which allows
sorting of PrintInstances for sequential printing in the order
they appear in Plater's object list.
reorder_by_three_exchanges_with_segment_flipping()
works, but it is excessively slow, with close to O(n^3) time complexity.
Commited, but not used in production until sped up.
Implementation of 2-opt pairwise exchange iterative improvement
algorithm with an extension to a chain of segments, where
the chain of segments may get flipped during the exchange operation.
The 2-opt exchange algorithm may be quite slow.
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.
producing better shortest path estimate than the "closest next neighbor"
heuristics. The new greedy algorithm utilizes KD tree for closest
end point search, and builds a graph to detect loops.
PerimeterGenerator newly uses the optimized TSP algorithm.
ExtrusionEntity has been refactored / simplified.
