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>

libraries/bezier-rs/src/bezier/mod.rs

@@ -31,6 +31,11 @@ pub enum BezierHandles {
 		handle_end: DVec2,
 	},
 }
+impl BezierHandles {
+	pub fn is_cubic(&self) -> bool {
+		matches!(self, Self::Cubic { .. })
+	}
+}
 
 #[cfg(feature = "dyn-any")]
 unsafe impl dyn_any::StaticType for BezierHandles {