In Path tool when dragging a handle, make Alt recover the opposing handle if it's not cubic rather than zero-length

[bakayu]

Partly closes #1870.

Path tool

When dragging a handle:

• Alt keeps the handles equidistant while pressed (instead of Shift in use now)
  • It also creates a matching handle if the other is zero-length or not a cubic Bézier

Also, fixes the following issues from the previous PR #2160:

• Issue 1
• Issue 2

[Keavon]

!build

[github-actions]

📦 Build Complete for c6b29a3
https://f8743a32.graphite.pages.dev

[Keavon]

There is technically one relatively insignificant problem but I'm going to let it slide in the interest of merging this now. Feel free to fix it in a later PR. If an opposing handle doesn't exist, because that segment is not cubic, pressing Alt works but then releasing the key puts it back in a way that permanently alters it such that it stays cubic and remains zero-length. So the net effect of tapping Alt is the permanent change from non-cubic to zero-length. It's subtle but not ideal. Thanks.