Make the Spline tool draw a polylines with a Path -> Spline from Points node setup

[Keavon]

Part 1

Listen to the task description in this video.

Part 2

This can be a separate PR.

1. Create an upgrade script to transform a single existing "Spline" node, in the graph of any existing documents people have today, into a Path -> Spline from Points node pair with the point data transferred over to the Path node's geometry
2. Delete the "Spline" node
3. Rename the "Spline from Points" node to just "Spline"
4. Create an upgrade script to rename the "Spline from Points" node in existing documents to "Spline"

Search the project for "document upgrade code" and you'll find all the comments relating to the upgrading scripts. The ones in portfolio_message_handler.rs, within the handler for the PortfolioMessage::OpenDocumentFileWithId message, are most relevant to read and that's likely where you'll be adding most or all the upgrading code to.