Write a rendering example demonstrating how to render multiple AXR documents at once

[jakepetroules]

Once axr/core#169 is completed, we should write another example app that demonstrates how to render two separate AXR documents at once.

The application should have two viewports side-by-side in a split view. Write it in Qt, making use of the QAXRWidget class.

Formerly axr/core#173