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

[Keavon]

• Part 1 (Change Spline tool behavior to use Path and Spline from Points nodes instead of legacy Spline node #2200)

  Listen to the task description in this video.

• Part 2 (Implement extending, joining, and creating new subpaths with the Spline tool #2203)

1. Make the Spline tool visualize the endpoints of each spline path just like the Freehand tool does (even if there are multiple spline paths because the Path node has multiple subpaths)
2. Allow clicking on the endpoints of a spline (on either end), when not drawing, to begin drawing a continuation of the spline
3. Allow clicking the start point while drawing spline segments to add a segment which closes the path given to the Spline from Points node
4. Allow holding Shift to begin drawing a new spline subpath in the same layer, instead of making it add a new layer (just like the equivalent functionality with the Pen tool)

• Part 3 (Add upgrade script to convert "Spline" node to "Path" -> "Spline from Points" #2274)

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.

• Part 4 (PR 1 of 2: Implement initial infrastructure for the Spline tool to join itself with other splines and paths #2269, PR 2 of 2: Fix Spline tool so its merging endpoints with other layers happens immediately, not after canceling #2319

  Merging path with splines, splines with paths, splines with splines, and paths with paths by joining their endpoints correctly. This is described as part of https://files.keavon.com/-/PunyUprightStagbeetle/capture_35_.mp4

[kliwongan]

Can I take this issue as my first issue?

[Keavon]

@kliwongan this is currently being worked on by @indierusty (here's the Discord thread). If there isn't a viable PR opened within a week, feel free to ask again if it's available then.

[Keavon]

@kliwongan I ended up adding a new part to this issue, so now it's a three-parter. Feel free to work on part 3, which isn't blocked by the previous two parts, if you would like. If you plan to work on it, please let me know when you'll be beginning.

[indierusty]

Implemented Part 1 i.e. make spline tool to use Path and Spline from Points node instead of just Spline node to make spline tool further non-destructive in #2200

[Sidharth-Singh10]

Fixed part of part 4 in #2269