Graphical editor for multiarrows

[epatters]

Generalizing the existing widget for (unary) arrows, create a widget for editing a "multiarrow" or "corolla". These represents the morphisms in a multicategory, hence have multiple inputs and a single output.

Here is a multiarrow with three inputs:

The inputs, output, and inner label should be editable.

This component will be part of the frontend implementation of discrete cartesian theories (#241).