Add molding segments to the Path tool

[4adex]

Partly closes #1870:

• Clicking and dragging without any modifier key, as per Sculpting / molding segments with the Path tool #615, should mold the segment.

Closes #615.

[Keavon]

!build

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[Keavon]

• Right-click and Escape need to work to cancel molding
• Alt should temporarily break colinearity and mold only the dragged segment, while also snapping the handles back to where they began before the molding drag began
• C should permanently toggle colinearity on the two adjacent anchors, without snapping the handles back like above (be sure to handle the case where they're each initially of opposite state)
• Hints should be updated for all of the above
• We can merge this but then move onto finding a better algorithm that suits our desired molding behavior better
• Please don't put any new code into Bezier-rs, find somewhere reasonable in the editor code to put that. Thanks.

[Keavon]

!build

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[Keavon]

I should update my request about C: it should do the same that Alt does now, except on a toggle basis instead of a held bass like Alt. Tapping C two times is equivalent to pressing then releasing Alt. But if it starts as already not colinear on both ends, then C does nothing and shouldn't be shown in the hints bar.

[Keavon]

• If you hover and then scroll with a scroll wheel or middle click drag to pan, the line that follows your cursor when hovering over a segment doesn't go away when your cursor goes out of range, until you next move your cursor by a pixel after panning. Please fix that to make things nice and polished.
• After testing it, I've realized that C adds more complexity than it's worth, so let's please remove it and just stick with Alt.
• When Alt isn't valid, please make sure the hints reflect that by only showing what's valid when it's valid.

Nearly ready to merge!

[Keavon]

!build

[github-actions]

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