Fixes:
* Various special cases around junctions on pins and dragging.
* Some rotations of endpoints resulting in a 45 degree rotate.
* Some cases where it was possible to get a line with neither
endpoint selected, and also substractive unselecting only one of two
selected endpoints unselecting both.
* Use line midpoint for sorting. Start and endpoints aren't
consistent in the order they appear on the X or Y axis. So,
we need to use the midpoint for our position for consistent
sorting when dragging groups of parallel lines where some
have the start and end reversed.
Other:
* Rename TEMP_SELECTED TO SELECTED_BY_DRAG. This is the actual meaning
of the flag, and should reduce confusion as
to when it should be used.
* Move usage of TEMP_SELECTED as an algorithmic mark to CANDIDATE
instead.
* Fix mistaken clearing of START_POINT and ENDPOINT.
* Move endpoint setting and clearing out of narrowSelection, and into
selectPoint and selectMultiple.
* Don't show dangling end warnings on new lines
e51ab86225