Implement Freehand Tool (#503)

* Implement freehand tool

* Address review comments

* Consistent ordering of tools

* Add hotkey N for freehand tool

* Tweak freehand tool hint

graphene/src/document.rs

@@ -474,7 +474,7 @@ impl Document {
 
 				Some([vec![DocumentChanged, CreatedLayer { path: path.clone() }]].concat())
 			}
-			Operation::AddPen {
+			Operation::AddPolyline {
 				path,
 				insert_index,
 				points,

graphene/src/operation.rs

@@ -45,7 +45,7 @@ pub enum Operation {
 		transform: [f64; 6],
 		style: style::PathStyle,
 	},
-	AddPen {
+	AddPolyline {
 		path: Vec<LayerId>,
 		transform: [f64; 6],
 		insert_index: isize,