Add Path tool support for Alt-dragging an anchor to pull out a fresh equidistant handle pair

[4adex]

Part of #1870

Path tool - When dragging an anchor:

• Alt (new since video was recorded), if held during the start of the click-drag, converts the anchor's handles to colinear and drags the handle pair instead of the anchor, keeping them equidistant unless Alt is released to temporarily return the opposite handle back to where it started before the Alt-drag began (which involves returning its colinear state back to what it was before). Should work with Tab to swap which is the dragged handle, so its opposite can be the one that's returned if Alt is temporarily released. Just Alt clicking but not dragging the anchor should remove both handles by setting them to zero length.

[Keavon]

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[Keavon]

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[Keavon]

Screencast with updated requests: https://files.keavon.com/-/StingyFocusedXiphiasgladius/capture_3_.mp4

And let's be sure, using this discussed strategy, to avoid this from crossing like this: https://files.keavon.com/-/BitesizedMixedBasil/capture_85_.mp4

[Keavon]

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[Keavon]

We need Alt-click (without drag) to collapse the handles to zero-length in the same way that double-clicking does. See the comparison:

capture_39_.mp4

[Keavon]

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[Keavon]

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