Rewrite Drag Point Tooling Using Splines

[freezy]

Unity now has its now the Splines package that could be leveraged for editing drag point-based elements.

For rubbers, it would be beneficial to implement additional features:

• Auto-snapping to "rubberable" elements (pegs, posts), so authors don't need to fiddle with the drag points at all
• Simplication of one-post rubbers (see Rubbers: For simple cases, point based editing may be over complicated.  #242)