Path tool should have a linear/quadratic/cubic toggle for selected anchors

[Keavon]

I need to design icons for these three modes that'll be displayed in a radio button arrangement (use text for now as placeholders). This will finally make it possible for the user to see whether an anchor point is currently set to linear, quadratic, or cubic and have control over that.

We also want quadratic segments to have their single handle connect to both its endpoints with a line overlay visualization, not just arbitrarily one of the two endpoints. This will help also clarify to users when a segment is quadratic, since its handle will visually being to both anchors.

Also, hitting the delete key should set handles to zero-length but not remove it, turning cubic into quadratic or quadratic into linear, as it does now. See this #code-todo-list task.