Make the polyfill match the WebXR Devices API as closely as possible

[TrevorFSmith]

Now that the CG has a plan for WebXR (including using the XR namespace) we need to make our polyfill as close as possible to the first version train (basically VR support) and separate out the bits that are experimental and for the second version train (the AR bits).

For v1 branch:

• Rename XRDisplay to XRDevice
• Restore stage coordinate system
• Add XRFrameOfReference with stage bounds
• Restore session setup
• Double-check that layer setup is draft consistent
• Implement all of the events
• Remove the need to sleeps

For vNext branch:

• Anchors
• Geospatial coordinate systems
• Additional session setup parameters
• Floor anchors (perhaps rename to foot level?
• Light estimates
• Point cloud