Add Pen Tool (#536)

* First steps toward pen tool, similar to spline tool currently.

* Broken WIP

* Progress, but still glitchy

* Improvements, a little farther

* Pen tool functioning as expected

* Merged master

* Fixed commit bug and overlay flashing

* Reordered import statements

TODO: Resolve issue with last segment losing its handle position on confirm

graphene/src/operation.rs

@@ -88,6 +88,14 @@ pub enum Operation {
 		style: style::PathStyle,
 		closed: bool,
 	},
+	AddShape {
+		path: Vec<LayerId>,
+		transform: [f64; 6],
+		insert_index: isize,
+		bez_path: kurbo::BezPath,
+		style: style::PathStyle,
+		closed: bool,
+	},
 	DeleteLayer {
 		path: Vec<LayerId>,
 	},