Path tool: sliding point insertion (#1581)

* #1578 sliding point works(1st approx)

TODO:
* don't move too close to the side points
* double click works incorrect?
* do we need to jump from segment to segment?

* #1578 disallow move slide point too close to side points

* fix double click + ctrl insertion

* #1578 select insertion point (except `ctrl` case)

* #1578 far depends on line width & more accurate seg finding

* #1578 insert point on most top suitable selected layer

* #1581 draw insertion point by overlay  + `Esc` abort

* #1581 sharp stay unchanged on double click

* #1581 fix incorrect handle of scaling

* #1581 `square` selection point & too close in px(instead of magic)

* #1581 bug fix: insertion point on unselection

* #1581 use `color: Option` instead of `SelectionType`

* Some code review, still need to review shape_editor.rs

* #1581 insert sharp point on a straight segment

Also correct insertion on quadratic segments

`ManipulatorGroup::have_handle` have such form because `handle = Some(self.anchor)` often used instead of `handle = None`

* Final code review pass

* Code review pass

---------

Co-authored-by: Keavon Chambers <keavon@keavon.com>

editor/src/messages/tool/common_functionality/snapping.rs

@@ -334,7 +334,7 @@ impl SnapManager {
 			let viewport = to_viewport.transform_point2(ind.snapped_point_document);
 
 			overlay_context.text(&format!("{:?} to {:?}", ind.source, ind.target), viewport - DVec2::new(0., 5.), "rgba(0, 0, 0, 0.8)", 3.);
-			overlay_context.square(viewport, true);
+			overlay_context.square(viewport, true, None);
 		}
 	}