Insert point when hovering on segment + Ctrl click to delete segment

[4adex]

Part of #1870

When clicking a segment (new since video was recorded):

• Clicking without any modifier key should split the segment where clicked. Currently this has the extra complication of entering an insertion mode where the point of the segment split can be slid within the segment and left click confirms it while right click cancels. We want to remove that extra step and just insert directly. An overlay should be drawn in cases where the cursor is hovered over the segment such that clicking would insert a split, it should be a short line segment (20px in diameter) running perpendicular to the curve.

• Ctrl clicking should delete the segment that's clicked. Currently a user would have to split the segment then delete the newly created point.