Implement the Pen Tool for Bezier curve creation

[Keavon]

The last major feature Graphite needs in order to create real art is custom Bezier curves, created with the Pen Tool and edited with the Path Tool (#82). See that issue for details of steps involved.

Some food for thought: https://medium.com/@trenti/the-mighty-pen-tool-6b44ff1c32d

Complexity: 3
Involves: Editor (Rust), Document (Rust)