Add option to add arrows to start, end, mid-points,
and to swap direction of midpoint arrows.

This functionality is similar to what PyDMDrawingLine
already has added.

-Add update button, saves and applies changes to settings
without closing the settings-editor window
-Add ability to swap point ordering in settings-editor
by drag and drop

Also:
-minor change to member variable name to be more consistent.
-minor change to comments