This change allows the chaining function to select edges from the same contour loop first, thus reduced rouge chain connections (connected different objects instead of chaining inside the same object first) and improved chaining quality.

This patch also included the default value change for chain split threshold (Now don't split by default to make initial result as smooth as possible). Maybe it can go into a separate commit? This value change is originally included with this feature as a intention of providing smoother "out of box" result, if needs to split I will then commit them as two separate patches.

This is also a feature that I forgot to put into master