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/subpath/structs.rs

@@ -117,6 +117,18 @@ impl<ManipulatorGroupId: crate::Identifier> ManipulatorGroup<ManipulatorGroupId>
 		std::mem::swap(&mut self.in_handle, &mut self.out_handle);
 		self
 	}
+
+	pub fn has_in_handle(&self) -> bool {
+		self.in_handle.map(|handle| Self::has_handle(self.anchor, handle)).unwrap_or(false)
+	}
+
+	pub fn has_out_handle(&self) -> bool {
+		self.out_handle.map(|handle| Self::has_handle(self.anchor, handle)).unwrap_or(false)
+	}
+
+	fn has_handle(anchor: DVec2, handle: DVec2) -> bool {
+		!((handle.x - anchor.x).abs() < f64::EPSILON && (handle.y - anchor.y).abs() < f64::EPSILON)
+	}
 }
 
 #[derive(Copy, Clone)]