Draft: [Corners LPE 5] Retain on joins

Corners LPE [SPLIT 5] retain on joins.

Is a bug fix, happends when you have a path with corners and other without. When you join the two separate items. depend of the order of selection extrme we can loose the LPE, instead preserve and add 0 with to non LPE one.

This isue affect more LPE but is a hardest issue to define whith LPPE has preference over others (for exampel when the two paths have different LPE. So I opt to keep simple and do for one LPE (probably the most used in terms of pen/node ones) Also for bspline/spiro no way to handle without flattening the LPE before join, because this LPE precise a predefined path to work, and alter the other one if we force move to retained path.

here all refactor bundled to test: !6232 (closed)