MNT: Cleanup drawing lines class-structure

[nstelter-slac]

followup to #1072

move shared code between line and polyline into base class, removes redundant code

also move polyline class up in file, so is right after line class (for easier reference imo)

[YektaY]

This update looks good, my only comments are with adding type hints.

Edit:

I have found a bug that crashes Designer. if you create a PyDMDrawingLine and then edit the arrow properties, before adding points, then designer will crash