Pen Tool

[dabbott]

We use the pen tool to manually create a path from individual points. The pen tool is fairly complex, so we'll break it down into a few smaller issues/PRs:

• Render control points: #107
• Click to select a point: #108
• Click to select a control point: #109
• Control point inspector: #110
• Drag points and control points: #116
• Draw new paths using the pen tool: #124