the PR fixes the following things:
- change the way centerlines between 2 lines are calculated. The current implementation leads to many bugs and even crashes (e.g. when the 2 selected lines are parallel ones of a square). There are different forum threads from the last 1.5 years.
The new endpoint line sorting is the one described here: https://forum.freecadweb.org/viewtopic.php?f=35&start=20&t=44255&sid=989a668890f954c13ef81e4a04ee6912#p501179
- as consequence the erroneous and misleading line end flipping can go and is removed (only used internally)
- when creating a new centerline, you see it immediately
- when creating a new or editing an existing centerline and press Cancel, the creation/editing is aborted
- fix crash when the 2 selected lines are bot horizontal and one tries to create a vertical centerline
- fix crash when changing the orientation of an existing centerline and the result would be invalid
- cleanup the code a bit
