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