undo "inline insertions/deletions" should undo setting of helix tick marks

[dave-doty]

Create this design:

Select "Edit→Inline insertions and deletions". It results in this as expected:

Now press Ctrl+Z to undo. It should go back to the first picture where the tick marks are 0, 4, 8, but instead it retains the tick marks as 0, 5, 9:

[dave-doty]

This also happens with paired insertions:

then inline:

Then select Undo:

But it's purely a View problem; the design appears to be properly undone. If you refresh the page, you see the original design.

[dave-doty]

This was a subtle issue to find. The cause was the memoized getter Helix.calculate_major_ticks returning a List<int> instead of BuiltList<int>; see issue #766.