Viewport

A semagrams app is split into several viewports.

The reason for this is that we might want to apply different affine transformations to different parts of the app. For instance, we might have UI elements in one viewport, and the main acset in another viewport, so that we can zoom out on the main acset without making all of the buttons tiny.

Currently transforms are unimplemented; however this was just the result of an incomplete refactor, and it should be straightforward to add them back.