Add snap and lock angle modifiers for handle dragging to the Path tool

[bakayu]

Closes #1870 partly...
This PR aims to close the following parts of #1870 :

• Shift locks the handle angle to 15° increments
  • (shouldn't interfere with Shift-clicking handles for multi-selecting points such as handles)
• Ctrl locks the current angle of the handle

[Keavon]

!build

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[bakayu]

@Keavon This PR is now ready for review!
Please produce a dev build for testing ^^

[Keavon]

!build

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[Keavon]

Please check off the boxes in the issue description for what's implemented now with this PR.

[bakayu]

By now, this PR has implemented all of the listed features. Updated the description as well.

Edit:
Though I'm still skeptical about this part-

• Alt keeps the handles equidistant while pressed (instead of Shift in use now) (Add Path tool support for the Tab key swapping to dragging the opposite handle #2058)
  • It also creates a matching handle if the other is zero-length or not a cubic Bézier

Please let me know if anything comes up during testing

[Keavon]

QA feedback:

• If the handles are not colinear, releasing Alt should put the opposite handle back where it started before Alt was pressed so the user can have the freedom to press Alt any number of times without worrying about affecting things by changing your mind and releasing Alt.

[bakayu]

QA feedback:

• If the handles are not colinear, releasing Alt should put the opposite handle back where it started before Alt was pressed so the user can have the freedom to press Alt any number of times without worrying about affecting things by changing your mind and releasing Alt.

Noted.
For now, I have reverted the commit that aimed to fix this, I will address this later in a seperate PR.

Updated the description to reflect what this PR currently solves. This much should be ready for reviewing and merging.

[Keavon]

!build

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[Keavon]

!build

[Keavon]

There's one more issue I found, but it can be fixed in the next PR you work on:

• If you begin dragging a handle with Ctrl already pressed, it uses the angle of the previously selected handle rather than the being-dragged handle.

[bakayu]

I will continue the work in a couple days, will fix it then.

[Keavon]

Oh and one more:

• If I'm holding Ctrl to lock the angle, then add Shift to snap it to 15°, then release Shift, it should return to its previously locked angle that was set when the currently held-down Ctrl was pressed down. This also needs to be true for the Line tool, which isn't currently the case. Also for the Pen tool, which currently doesn't even let you add Shift while Ctrl is held. What I'm describing should be its own PR.

Update per your feedback on Discord: instead of restoring the old angle, Ctrl being held down should make the addition of Shift be ignored, thus keeping the locking angle and giving it precedence over the 15 degree angle snapping, until Ctrl is released.

[github-actions]

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